Whumper who's practicing on their whumpee. They're learning, testing out their tools and their strength.
Which cuts leave scars, and what kind? How deep do they have to go, what kind of blade to get what kind of results?
How hard to hit with weapons or fists to get which kind of bruises, what is too much, and what's not enough?
How to tie restraints, how long can whumpee last in certain positions.
They test and try, and make notes, inspect the results. It's clinical and intimate and invasive. It hurts and whumpee screams and whumper notes down which kind of screams are their favourite.
They learn how to patch up wounds, how to prevent serious injury. Whumpee is burned and scarred and not one inch of their skin, by the end, is unmarred. There's no love, but there's such appreciation for their test subject, praise at taking it so well, at helping Whumper learn. They shiver and shudder under the touch of their captor-- a captor who never answers why they're doing it at all.
And then one day Whumper declares themself ready, they've learned everything they can.
They bring home their favourite subject, their ideal captive, the person they've been dreaming about doing all of this to, once they were skilled enough. And Whumpee watches it all happen again, hears the screams and the sound of the tools. Only it's with infinitely more precision, much less scarring, there is no random pain. They were the trial run and now Whumper can carve into someone else and aim for perfection.
Whumpee's job is done, and they're just left to clean up the mess that this new-found knowledge leaves behind.