In relationships, power flows to the point of least desire. That is to say, ultimately, the only real power in a relationship is the power to walk away - if one is not willing to do that, one is at the mercy of whoever in the relationship *is* willing to walk away. All other power in a relationship is ultimately given - the only power one truly *owns* in a relationship is the power to walk away. Everything else is something your partner gives you, or something given to your partner.
NB: This is not triggered by issues in any of my relationships. Mostly, this comes from a) having just finished a book on power politics, and b) being madly caffinated at 4:23 am. I am willing to discuss why I am wrong, in this thought.
Editted to add - the only power someone has over you, then, besides the power to leave, is the power you *give* them. This exchange is made explicit in D/s relationships, but I think it applies to *all* relationships.