In BDSM games it is common, to use a Safeword, to end the action immediately or to decelerate the game. Common practice is the Safeword Mayday or the traffic light system.
Green means: Everything OK. It is horny! Keep it up!
Yellow means: It's getting dicey. Please do not firmer!
& Red gives me the sign, that the action just is too hard for you and I stop immediately.
If you have a gag in your mouth or I find myself with my ass on your face, then knock off like a wrestling match. But when your hands are also fixed, then shake your feet or make yourself felt on a different way :)