It had been an idle question on Spike's part, barely an idea at all.

"How did the mirror pool work, anyway?"

Twilight had found it unacceptable that she didn't have an answer so she rolled the boulder from the cave, and got to sciencing. She found that the pool cloned whatever was placed in it in exchange for a bit of organic material, then gave it a jolt of extra energy. A strong enough jolt to raise the dead, and the cloning process wasn't picky about what it started with.

It's been a few months now, the hubbub and hullabaloo has died down, and the ponies of Ponyville use her technology extensively, particularly the cutie mark crusaders.