

How does Force Drain work?
How does force drain actually work, because it seems like a pretty easy instant win move in a fight.
Darth Nihilus and Darth Vitiate are the two most well known users of force drain, Nihilus eating planets and Vitiate absorbing all life on Ziost.
But I was wondering if its so powerful why don't they just use it to win every fight?
Unlike force lightning you can't just block it so does it have something to do with how powerful you are in the force or do they have to charge up before draining something?