Tremendously slowed down so you can try to hack at the question; How did Rarity do that!?

Apparently you can slip shorts on over your head. The only plausible explanation for how this all happened IMO is:

0. Outfit is turned inside-out

1. It’s thrown on in such a way that impact turns it right-side out.

2. With enough speed and force, Rainbow’s body was sent through one of the shorts’ leg-holes.

3. As Rainbow is forced into a crouch from the force of impact, her skirt and short-legging-things are launched to the floor. Thanks Rarity, for pants’ing your friend.

4. While the shirt slips on perfectly over the top as you normally put a shirt on, the new shorts Rarity is launching go all the way to the floor, under her feet, where her foot is slipped through the so-far-unused leg hole, then

5. With a recoiling motion, the shorts are pulled up in the last frame we see Rarity’s hands near Dash’s feet.

Well, that’s the best explanation I got Xp