In my sessions I use “safe words”, which is the norm in western BDSM, but seems quite uncommon in Japan. A “safe word” is a word that is irrelevant to play that means “STOP”. It should be used by the submissive (and in some cases the dominant!) in any situation that they feel too uncomfortable or painful and they want to pause or stop the current play immediately. I also have a word that I have my slaves use when they are approaching their limit, which acts as sort of a yellow light.
The reason for using this word instead of simply saying, “ouch!” or, “stop!” is because the base of SM play is the power dynamic between the dominant and submissive. It can excite both parties when the submissive is begging for mercy. However, it is absolutely necessary for there to be a way to signal to your partner that your limit has been crossed.
I never get angry at a slave for using the safe word. Each person has their own limits and our limits vary by day and our current conditions.