Huh, sounds like Anna may be using potential energy to power her ultimate attack...
Quick physics lesson! When a ball rolls down a hill, it's giving off kinetic energy. But when it was sitting at the top of the hill, then that kinetic energy isn't happening. Instead, you have potential energy, the energy it WOULD be giving off if it were rolling down that hill.
Then in comes Anna, who takes this not yet energy, and uses it to un-exist everything she wants gone. Basically, she's GETTING ENERGY FROM NOTHING AAAAH!