Rainbow Dash remembers how once, before Pinkies birthday, Pinkie Pie managed to keep up with Rainbow Dash, even after she had a three second head start before Pinkie started chasing her.

Feeling like her title as fastest mare in all of Ponyville, is being threatened, Rainbow Dash challenges Pinkie Pie to a Fastest Mare Alive Showdown [also known to ponies who aren't in The Fastest Mare Alive Official Club as a race.] And as the rule book for a Fastest Mare Alive Showdown states, the pony being challenged gets to decide when and where it's being held.

Rainbow Dash really hates that rule.