Start with spooning – and we’ll say you’re both lying on your left side to do this move. Enjoy some cuddly spooning sex together to warm up.
Then he moves his upper body back off her so that his left hand can get a nice handful of her hair. The key here is that the more hair you get, the more the pulling is evenly spread across her head = a firm sensation rather than a painful one.
Then if she puts her right wrist against her tailbone, his right hand can then hold her wrist against her tailbone. Do not pull her arm up or further back from her body as that can risk serious injury. Just press her wrist against her tailbone. You’re not actually twisting her arm up behind her back, she cooperates to get into a position where you are both pretending you are holding her against her will.
If the wrist against her tailbone is too much of a stretch for her comfortably, you can always hold her by her wrist against her hip.
Then get a fair bit more vigorous in the thrusting department. As always, consider “ow!” or “stop” a safeword. Maybe not “no” though… talk about that first though.
Just try it out together. If it works for you, then great. If not, at least you tried something new which is still a good thing.