Most people, even those in the lifestyle, simply see this as the submissive giving control of his or her body over tho the dominant. While this is, in fact, true, this does not constitute an exchange, but rather a gift. The other side of the coin which is often overlooked is that the dominant also gives control to the submissive. (For simplicity's sake, I'm going to use the masculine as the dominant and the feminine as the submissive.)
Think of it this way. You are in an office. You are the Boss's right hand man. You present ideas to your boss. She has given you the control, the ability, the duty to come up with ideas and to institute them - but only based upon her
This, in essence, is a TPE. In actuality, the Boss is the submissive. She listens to the Dominant's ideas, schemes, plans, desires. He, then, has given her control of agreeing with his ideas or vetoing them. She has given him control of coming up with the ideas. Thus, an exchange of power.