Normally I'd say everything is based on trust but that's not necessary true. There are many subs and slaves out there that love absolutely what some people do to them, doesn't matter how humilated or used they are even if the person dominating them doesn't really give a crap one way or another.
It doesn't matter if they know what is going to happen or are asked if they mind, they simply do what they are told and end up loving it because it's so much of who they are at their being.

Balance tends to be when each person enjoys their role and understands what is expected of them, they know what it involves and what may come from it. The balance can be changed when one person starts to act against what the other expects, be it a dom going switch or submissive in part or the sub starts to get more aggressive to the point she's not really being what she was.

Basically going against what each other expected and did all this for to begin with. If your not getting what you wanted thanks to one changing the balance is altered and it begins the problems so many have.