Just as in the vanilla world... the contract describes responsibilities, duties, recourses, penalties for non-performance, and how the contract will or can be ended.

In the vanilla relationship world, a lot of that "contract" is culturally understood, and often ends in a marriage license which has more cultural and legal processes and understandings.

In the world of bdsm, it is no different save that more is discussed, (or up for discussion,) and more is specifically specified, whether orally or in a written agreement.

That all said, you (hopefully) go into the negotiations equally and you enter into an agreement, whether for a couple of hours at a play party or a whole hog 24/7 relationship.

Who has the power, while "under contract" is based on the couple and their agreement. And the fact that either can withdraw is irrelevent. It's just another point within the agreement.