Communication is most important. Giving someone that much power over you is dangerous. Communicate beforehand and set the rules and boundaries. Yes, even a slave you have boundaries. Like can you be beaten so bad you need hospitalization? No? That's a boundary. Make sure you talk through EVERYTHING before you commit. Also communicate and agree to a way to exit the situation, revoke consent. At that point all activities stop and anything that happens after can be considered assault and/or rape. Also have a friend that knows what's up and a way to signal them when you are in trouble. Things can get out of hand quickly so be careful.