I suppose the most obvious answer to the question is the sub. He can say stop or use the safe word at any stage and then everything stops. The scene ends. But doesn't that change when the scene is with somebody you care about, that you value deeply as a human being and person and not just as a role? Then doesn't the power dynamic shift almost totally, if not completely to the Domme?
As a sub you can feel a very deep connection, almost as if she's inside you. it makes you want to push harder and harder, to take more pain and humilation because you don't want to lose that connection, that sense of total freedom and release. Theres almost a conflict. you feel yourself wanting to use the safe word but something stops you. I can take more you tell yourself. The pain is worth it if it makes her happy, if that look in her eye remains. And that, I believe is where empathy from the Domme is essential, the ability to read a person. knowing that it is time to stop and then making the sub feel that she felt the connection too as he slowly comes down. she takes power but she also gives it back.