Informed consent and safety
When engaging in role play, kinky sex or bondage you may like to consider discussing what you like, don’t like and your personal limits. Having a clear understanding in advance is very beneficial to both (or all) parties. Writing this in a document and signing it goes some way to protecting yourself. Example of areas to consider are shown below:
Level of pain
Bruises and marks
Ties and restraints
Safe sex
Anal, oral and sexual preferences
This is only a very small list of what could potential be very comprehensive.
Safe word
That hurts, stop that, ouch etc are not ideal ways to communicate during kinky sex. They are not clear and also not erotic. You should consider using a safe word that if said everything STOPS, no mater what. Then the rest of the time you can scream, shout, swear without interrupting the play. A more advanced consideration is using a traffic light system.
Red: Stop everything
Amber: Too much, ease off a bit
Green: Used after amber to mean that's okay