DOMINUS saidSuperb muscle control. I met a big muscle bottom a couple of years ago whose ass made my dick feel like it was getting a shiatsu massage every time I hit it. Awesome!
One of the best feelings I can think of or imagine is pleasing my top partner. I would think a combination of squeezing and releasing as implied above and reverse humping movements, i.e., the bottom man moving his butt up and down as well, maybe timing it with his top's thrusting actions. Having strong abs might help. If you want to test this theory out on me, please let me know