So the whole “tie Whumpee to a table” trope, where their arms are out to either side and they’re strapped down so tightly they can barely move. It’s used both for active torture and just keeping them in place when Whumper’s not paying attention to them.
When used with the later intent, put a weight on Whumpee’s chest. Nothing insane that will crush them, but something heavy enough to make breathing difficult, for them to feel like their ribs are going to snap under the pressure. It’ll occupy them when Whumper isn’t there to torture them, and it works especially well with Whumpee’s who have claustrophobia or cleithrophobia, the fear will make the situation seem so much worse.
Alternative/additional places weights can be placed: against their stomach, hips, thighs. I guess it depends on how they are restrained, and the structure of the weight you intend to use, but there’s a lot to play with there.
Have Whumper stay nearby for the first few trials, though. Make sure they’re listening for any sort of ragged breathing, and can intervene if Whumpee’s actually being crushed. It might take a few experiments to find what works best, each whumpee is different after all. Or don’t. Have them leave and come back in an hour to a half-dead Whumpee with fractured ribs. That’s one way to do it.