One of my favorite physical triggers ever is the "scruff" trigger. It's where you suggest to the subject that when the back of their neck is grabbed, they go limp. It's so fun to watch them slump and blush. Works wonders for discipline, too, because it's immediate.
Sub getting a little overwhelmed and bitey during sex? Scruff that pet and whisper teasing taunts in their ear while you continue to use them.