We call it power exchange. Usually I see it defined as a relationship where a slave gives up their power to their master. I think this is faulty thinking. What a slave gives up is authority over their power. A powerless employee is useless, a powerless soldier is useless, and a powerless slave is useless.
A slave is an extension of their Master’s power, augmenting it and amplifying it. It works the other way as well, the Master’s power amplifying and augmenting the slave’s. The relationship is symbiotic, the whole being more powerful than the parts individually.
Now, I like analogies and finding examples in the wider world, especially fiction. I recently finished listening to the Lords of the Sith audiobook. Perhaps Emperor Palatine and Darth Vader are not quality people to emulate, nor is the adversarial nature of the Sith’s Rule of Two a healthy way to conduct relationships, but their’s is definitely worth studying as an example of an authority exchange relationship.
Vader is unquestionably powerful, his power waxing during his apprenticeship to his master. However throughout the book, the Emperor tests Vader’s loyalty to him, which also tests his own authority over Vader. Like all in service to another, Vader isn’t perfect and fucks up in his own way giving cause for the Emperor to punish him (the details of which should, again, not be emulated in a healthy M/s relationship). But in doing so the Emperor increases both of their power and strengthens his authority over the former Jedi.
Stepping away from fiction now and back to reality now. As I am currently single, I have authority over my power. I get to choose how, when, and where I spend my time, resources, and energy. I choose how increase my power through educational choices, strengthen relationships, and, well, a billion other examples throughout the day.
In an M/s relationship, I never lose the power to do these things, but I do give authority to another to control how, when, and where that power is exercised. I never lose the power to jump, but when she says jump, I do so. As an extension of a Master’s will, she wields the power as her own and is also able to lend me her own power to be used under her command and authority.
I will grant that when I see actual existing, healthy, M/s relationships, I see authority exchange more often than power exchange, even if those in them use the latter terminology. However, I do think words have power (heh) and the distinction is an important one.