If I had answered this before, I would have said a sound-activated electro stim. Similarly, anything that lets a domme press a button to shock me, squeeze me, lift me off the ground, or stop me from moving would be wonderful. I'd love to be threatened with a button. I can think of a few other things too. One is something to completely immobilize my head, like a head-sized frame with stretched latex to hold my face. Another is big, heavy, metal cuffs. Of course rope or leather cuffs will do the job just as well, but I'd enjoy the look and feel of big metal cuffs on my wrists, ankles, and neck. My last idea is probably a stretch, but I once saw a very intense breath play scenario where a domme used a respirator and a gas mask to control exactly how much the sub could breathe. There are safety concerns with prolonged breath play like that, but I've never seen anyone so completely helpless. That's real power!