Phasma and Hux have an arrangement: when he starts to buckle under the stress, she hurts him until he can't take any more. He doesn't resist -- total submission is part of the deal -- and doesn't get to decide when it happens.

+ casual dominance gestures, like grabbing his throat, manhandling
+ caning/impact play/breathplay
++++ trust
+ Phasma is cold and detached during the discipline
+ she stops when she thinks he's had enough, not when he begs or cries (+ she's an excellent judge of his limits)
+ they both get something out of it
+ sometimes there's a sexual component, but that's not the goal
+ aftercare (+for both of them)