Good lord, you guys explained it better than I could have! Thanks~ <3 

But yes. I didn't imagine for Juliette to have some unlimited surplus she can draw from at will. Boosts or no, she gets tired like every other person and her energy depletes at a normal rate. I think "magical adrenaline" hits it on the nose; she does it in bursts because it gets taxing and prolonged use can have damaging effects on her body. That's where precision and timing becomes important, like Zurnt mentioned. She uses her magic sparingly and in conjunction with her skills as an acrobat to make the most of it.

Haa, sorry I couldn't be as scientifically eloquent as Rex. X3

Edit: [@rextremendum], I have a feeling that when Thomas talks science-y to Juli, everything will just go over her head and she'll come out of it with a headache. I wonder if he would get frustrated at her complete ignorance over the nature of her magic.