Author's Note
3 AM Trash. You might notice ratings disabled for this story. I wrote this for a friend's birthday and figured I'm not looking for upvotes or downvotes here. I just want them to enjoy it. 
Edit: They loved it!
Flash Feather
Summer, surprisingly enough, was a busier time at the Golden Oaks Library than other seasons. Thanks mostly to Cheerilee and her summer reading lists, each day Twilight was pleased to find at least one student nosing around the shelves, even if they were only searching for a copy of the thinnest, easiest book they could find to read. Still, though, it was Twilight's job to make sure those books could be found, and it was a job she absolutely cherished.
"Perfect!" Twilight squeaked, clutching her hooves together at the beautiful sight that lay before her. Twenty-two. There were twenty-two books on every shelf and every last one of them had been alphabetized. Each book was exactly six inches tall and each book sat exactly two inches in from the edge of the shelf for easy access! She had really, really outdone herself this time. She took a step back, looking over her achievement fondly before letting her gaze drift towards the window. The sun sat low in the sky and if Twilight had to guess without looking at the clock, she'd wager it was four-thirty, maybe five. That meant Rainbow Dash would be finishing her Wonderbolt's Training for the day and be arriving shortly to inevitably make a mess of things. Normally the spirited blue Pegasus didn't crash, but Rainbow was always hyped after she'd finished a training session and Twilight was certain somehow, someway, the perfectly aligned shelf she'd spent the afternoon painstakingly putting together would-
From the front door burst a blazing blue blur that came barreling in at break-neck speed. It slammed into the bookshelf, toppling an avalanche of heavy narrative and characterization onto the Pegasus pony below. Of course.
"Oh good. You're here," Twilight sighed flatly, meandering over to the shelf to see if she could spot Rainbow Dash under the numerous chapters she was buried under. Rainbow Dash, still in her uniform, tried to tug herself from the collection of fables, shook out her fur, and flashed Twilight with the widest smile she could muster.
"Hey, Twi! Sorry about that! We were trying out this amazing new formation and I really wasn't expecting to catch Fleetfoot's slipstream the way I did! But, uh, yeah! I'm here! Hey!" She smiled. Twilight stared.
"Hey," she groaned, helping to tug Rainbow free of the books that still surrounded her. She glanced at her shelf and rolled her eyes. Most of the books had managed to be knocked off and those that remained were no longer perfectly aligned. Still, not the worst thing in the world. It just meant she got to do it all over again. But she had more important matters to attend to now that Rainbow Dash had arrived.
"So, what's this about a spell you needed me for?" Rainbow squeaked, following Twilight's gaze to the shelf and the numerous books on the floor. She grinned widely. Regardless of Rainbow's attempt to change the subject, Twilight's gentle smiled returned and she gestured towards her desk where a lengthy scroll of parchment lay waiting for her.
"Well, I've been thinking lately. With all the monsters we run into and the situations we face, it would be nice to come up with a few new defensive spells and I was hoping you could help me test one or two of them," Twilight explained cheerfully, trotting towards her desk where she gripped the scroll with her telekinesis. She unraveled it and took a seat in front of Rainbow
"Really?! COOL! So, like, fireballs and stuff? Lightning and maybe like, ice rockets?!" Rainbow squealed. Twilight rolled her eyes and shook her head.
"Actually, none of the above. I was hoping to find more non-violent solutions to our situations. I don't really want to launch a fireball into a monster, no matter how terrifying it is," Twilight said. Rainbow Dash's smile faded, but she nodded her head all the same. While it would have been cool to have Twilight slinging fireballs, she could understand, and more importantly, respect Twilight's desire to see conflict through using non-violent means.
"So, what? You're testing the spells out on me?" Rainbow asked. Twilight flashed sheepish grin and timidly nodded her head.
"Err, y-yes. Well, you're the toughest pony in town, so I figured if it could take you down, they might be worth keeping around," Twilight explained, scanning Rainbow's face as subtly as possible. The Pegasus's chest swelled and Rainbow marched towards the center of the room with a brash and arrogant grin distorting her muzzle. Hook, line, and sinker.
"Well, obviously I'm the toughest! They're non-violent, so they won't hurt, though, right?" Rainbow sputtered, only the slightest hint of nervous tension in her voice. Twilight immediately shook her head.
"Hurt you?! Rainbow, N-NO! I'd never cast a spell that would hurt you or any other pony! That's the whole point of this test!" Twilight stammered. Rainbow's ears immediately drooped and she winced. That was silly. No. Of course Twilight wouldn't hurt her. She knew better than that and sighed.
"S-sorry Twi. Yeah, right, I know. That was dumb. But, uh, if it's not gonna hurt, go on! Hit me with your best shot!" Rainbow barked, immediately throwing her chest out again and taking on the most heroic pose she could think of, spreading her wings out as far as she could, and perking her ears to look bold and brave. Twilight had to admit, she pulled off the image rather stunningly.
"Don't worry about it. Alright, for the firs-"
"Yadda yadda yadda! Let's just go already! Whatever it is, I can handle it! Toughest pony in town, remember?!" Rainbow laughed. Twilight grinned.
"Well, if you insist," Twilight said, ducking her head behind the scroll. Rainbow watched, twitching with anticipation ready to shrug off whatever spell Twilight was about to throw at her. The alicorn's horn was beginning to glow. Rainbow tensed but she didn't notice the rope until it was too late. It burst out from the ground and slithered like a snake, swathed in a purple aura and moving with a speed that even surprised Rainbow Dash. For all of her super speed, the Pegasus was caught off guard and the rope securely wrapped around her ankles.
"H-Hey! What gives?!"
"Well, you said to go ahead, and I-"
"No, no, no. I mean this? This is it? Some dopey rope? I can..nnghh..pull out...ugh...of this easily....hhhnng..." Rainbow groaned, twisting and tugging at her legs valiantly, but to no avail. She gave a hot snort, and strained, doing her best to free herself from the mystic restraints.
"I...I'm...I'M GONNA BREAK THIS ROPE!" Rainbow screamed. Twilight rolled her eyes.
"You can't break that rope," she said nonchalantly. Rainbow growled and gave it her best shot. Really she did, but she wasn't going anywhere. Twilight ducked back behind her scroll and her horn began to fizzle again, popping with magical purple energy. Nervously, Rainbow watched, eyes locked on Twilight waiting for the next spell to occur. Some paralyzing ray, no doubt, or some sort of transformation spell. It was then, Rainbow felt the first gentle wisp of something across her side. Even through the Wonderbolt's outfit, she could feel it clear as day. Her eyes widened and she began to squirm, trying to catch sight of whatever it was that was touching her, but all she managed to see was a hint of glowing purple out of the corner of her eye.
Then, a second one. Something else was touching her now, delicately brushing along her other side, back and forth, like a skilled artist with a brush. Masterstroke after masterstroke, Rainbow squeaked quietly, realizing Twilight's plan entirely too late.
"H-Hey! Cut that out!" She snarled, biting back a fierce giggle threatening to rip its way from her mouth. For a brief moment, just a hint, really, Twilight leaned out from behind her scroll and griiiiiiiined. Uh oh.
"I call it 'Flash Feather' and I think it'll be a great help to incapacitate our foes without hurting them!" Twilight chirped brightly, the pride riddling her voice not lost on Rainbow Dash. She grunted and arched her back, squirming left and right as the arcane touches continued. For an instant, she caught sight of one of the objects. Glowing a gentle shade of violet, it appeared to be a long, thick goose feather. It crept up along her side to her neck where it spiraled and twisted and then back down again. Carefully, Twilight monitored the situation, taking notice of what made Rainbow Dash squirm the most and where. In full control, she manipulated the feathers, driving them towards Rainbow's legs and sides, brushing faster now, stroking with intent.
Ticklish as she was though, Twilight had only managed to produce a few measly chuckles from her victim. If she wanted to make the spell work properly against the bigger badder beasts of the world, she needed to up the ante. Now, where else would a Pegasus be ticklish? The way Rainbow was pressed again the floor by the ropes, meant her hooves were out. The whole town knew that Rainbow had a 'thing' with her hooves, and for the longest time, Twilight was willing to bet she was ticklish. A shame she couldn't test that theory out now. Where else...what about...
"Hmmm, let's see, how about this?" Twilight cooed, the feather gliding up along Rainbow's back where it stopped at her wings. Delicately, the feather began to twist and wriggle with surgical precision against Rainbow's wingpit. Immediately the Pegasus gave a squeal and did her best to buck herself forward, despite the ropes holding her down.
"WAHAHAHAHA! N-NO! NOFAIRNOFAIR!! Rainbow shrieked, arching her hips and squirming wildly against the ticklish onslaught. Twilight shook her head, watching quietly.
"I thought you said you could handle it? Just a few feathers to take down the toughest pony in town?" Twilight chided and immediately, Rainbow pulled in a deep breath and stopped laughing. Twilight frowned. More resilient than she had anticipated. No matter. There were other spots, and now that she knew the wingpit was one of them, she could make use of that information in the future.
Slowly, Twilight pulled herself out of her seated position in front of Rainbow and began to circle the bound girl like a shark. Where else, where else, where else? Oh? Twilight paused. Now that was interesting
"Twilight? W-what are you doing?" Rainbow asked, her voice wavering, but under control. The alicorn smiled sweetly.
"I never noticed the buttons back here. Your uniform has a drop flap?" Twilight asked. Rainbow gave a snort.
"Y-yeah? How else do you think we get them on?" she grumbled and squeaked at the sudden breeze that cascaded across her upturned rump. Twilight chuckled quietly, watching plump blue buns bounce into view. Somepony had been indulging on after show sweets, it seemed. Unfortunately for Rainbow Dash, the bared, chubby buns she sported were the perfect targets for Twilight. She conjured a single feather and without so much as a hint of warning, drove it gently along the cleft of Rainbow's bottom. The effect was instantaneous. The bound Pegasus barked with laughter, rolling her head back with a helpless squeal and immediately began shifting her hips and wiggling her rump from side to side.
"EEK! N-NO! Cutitout! ohmigosh!" Rainbow sputtered, managing -just barely mind you- to form a coherent sentence through her unbridled giggling. Not that Twilight cared. She perked her ears thoughtfully and flashed another sickly sweet smile.
"What did you say? Didn't quite catch that?" Twilight teased. A second feather appeared, this one slowly beginning an outward spiral from the thickest, fullest part of Rainbow's right cheek. Rainbow hollered, rocking back and forth in her bindings and wildly shook her head.
"HAHAHAAHAHAHANOTFAIRNAWHAHAHAHAHA" A third feather was summoned from the ether. This one taking Rainbow's left buttcheek to task, brushing up and down like a paint brush. The feathers working at her sides pulled away and relocated to her rear.
"HAWHAWHAWHAWHAW NOTTHEBUTT!! EEKEHEHEHEHEHEEHEE!" Rainbow squealed. She was laughing harder now. Much much harder than she'd ever laughed when Twilight teased her wingpits. The feathers were merciless in their approach, dancing a merry jig along the bouncing behind underneath them. Rainbow's cheeks clenched and relaxed, clenched and relaxed, an involuntary response to the wicked spell Twilight had thrust upon her.
"Aww, come on Rainbow! This is for science! I'll be able to write a whole paper about your ticklish rump! Coochicoochicoo!" Twilight teased, watching the first few tears of laughter stroll down Rainbow's cheeks. The fat behind arched into the air and dropped, bouncing boldly in any direction it could to escape the feathery torment. Twilight wouldn't let that happen though. Like puppets on a string, she easily manipulated her magic to engineer more peels of laughter from the Pegasus.
The tight little dark divide of Rainbow's cheeks seemed like the most sensitive spot, so that's where Twilight focused the tips of the feathers. They flickered up and down, entirely fixated on the oversensitive cleft. One of them worked their way upwards to the dock of her tail. Just at the spot the 'Y' of her cheeks started and decided it would trace over that 'Y' again and again and again. The two feathers on either side, left and right, stroked in parallel symmetry, while yet more were summoned. One gliding along the outer contours of Rainbow's bouncy blue bottom while the others returned to swirling in teasing spirals at the apex of either cheek.
"T-TWI! PUH-PLEAHEHEHEHEHE!" Rainbow sputtered helplessly. The way she rocked her hips was almost hypnotic, and more than one Twilight caught herself staring at the chubby cheeks Rainbow was showing off. Perhaps it was time to finish things before she got too distracted. So far, Rainbow hadn't seemed to be able to break her bonds, a good sign for sure, but Rainbow's energy hadn't given out. She twisted and bucked, squealed and squirmed, the feathers raged and the magic surged, but still, she kept going.
"Hmmm...how about here..." Twilight whispered. One of the feathers twisted about until it was vertical, matching Rainbow's crack almost perfectly and inserted itself into the shadowy ravine. Here it did its most fiendish work, bypassing Rainbow's rump all together to reach for slightly lower targets. The tip flickered back and forth in a gentle, maddening tickle, and Rainbow shrieked. She threw her head back and thrashed, eyes bulging at the brazen strokes the feathers made against her most personal places, and sensitive spots! They were relentless, dipping in deep between her cheeks and fluttering with a fury of magical strokes. She rocked herself back and forth, but the feathers remained persistent in their quest to tease her chubby cheeks.
Each stroke was carefully calculated, each terrible flicker contained a nights worth of planning and deliberation. It was maddening, to say the least, but the most embarrassing part was the feathers that had managed to slide inside of her suit. Down the drop flap to tickle just under her cheeks and wickedly between her legs. It was that feather, among all the others, that would do her in. Rainbow was sure of it.
"T-TWI! C-COME ON! NOT THERE!! I-IM GONNA WET MYSELF!!" Rainbow squealed, her voice loud enough by now to bounce off the walls of the library in a gentle echo. Not that Twilight could hear it over the sounds of Rainbow's squealing giggles. Of all the places on Rainbow Dash, Twilight would have never guessed tickling her behind would have such a reaction. She really did need to get started on that paper. She rolled the feathers in slow rakes, gliding them across the cheeks once, twice, thrice, and once more for good measure, before they returned to teasing her crack and parading around her chubby cheeks like they owned it. Whatever resistance Rainbow was attempting to show faded with the feather down there. In 'that' spot. There was no hope in keeping herself dry with such a fiendishly conjured plan, and deep in the depths of her mind, Rainbow realized she wasn't able to hold onto her dignity any longer. The feathers had won.
"T-TWI!" Rainbow shrieked, but it was already too late by that point. The poor cyan Pegasus turned from blue to scarlet in moments, and her uniform turned from blue to a much much darker shade underneath her. She squeezed her eyes shut, giggling madly, but Twilight had gotten all the info she'd needed. The capture had worked, the feathers too, a handy distraction, but it had not incapacitated Rainbow like she'd hoped. Embarrassed her wildly, yes, but not incapacitated. She glanced over the giggling mare, the feathers still dutiful. Twilight sighed and with a single hoof made a dismissive gesture and the feathers vanished from sight all at once. The ropes melted into the floor, dissipating like steam and Rainbow slumped forward, rump high in the air, tail cascading down her back and an exhausted look twisted her visage. She gasped and panted, trying to catch her breath and meekly looked up to find Twilight already scribbling wildly at her desk.
"That was very helpful, Rainbow! I gathered a lot of valuable data with that spell! I'll need some time to process this and look over my results, so do you think you can come back next Mond-?" Twilight asked, but discovered she was talking to herself with the front door hanging wide open. It may not have exhausted Rainbow quite as much as she'd hoped it would, but to have the knowledge Rainbow had fled at the idea of another encounter? Twilight smiled quietly to herself. She might be onto something here.
