From the source:
 
“Happy birthday, ice1517!  
This one is for you. Sorry if it’s not very good.
 
Something a bit different from my usual fare. Please don’t expect or ask for more. This is a one-time thing, for Ice’s birthday only.
 
I guess I made Rarity’s magic a bit too powerful, maybe? Eh, it works for the scenario.
 
Also, this is actually the first time ever you get to see how I imagine my humanized unicorns do spells. A spectral horn appears on their  
forehead to channel the magic. Pegasi work similarly, with spectral wings appearing when they want to fly. And a similar magic aura would  
surround the limbs of an earthpony when they perform feats of strength, athletics, or connect to nature or somesuch. “