Safeword.

I was recently required to take a self-defense and conflict resolution (i.e., Loony Wrasslin') class for work. Before the class they informed us: "Since this class will involve physical contact and roleplaying violent scenarios, if at any time something hurts you or you're uncomfortable with a situation, just say 'blue' and we'll stop immediately and make sure everyone's okay."

I tried very hard not to audibly snicker like a sixth-grader, but I have no doubt my smirk was visible from the Moon.

And I'm very lucky that I never needed the safeword (actually, as younger and stronger and way more gung-ho than most of the other participants, I had way too much fun tossing my coworkers over my hip repeatedly), because I spent the class in mortal terror that I was going to yell out "red" instead of "blue."