When starting a new game, you should
always establish the rules. D/s play is very intense. It plays with emotions like a rollercoaster plays with our sense of balance. And it can be quite thrilling.
At the very least you should establish a safe-word. This is a word that is not generally used within the normal play of the game. It therefore stands out. A standard example would be a spanking scene. Sometimes the recipient shouts "stop" and "no, no" as part of the ambiance of the play. But if they were to say "Red light", it would stand out.
For most purposes, traffic lights are an excellent indicator. We all know what they mean on the road. They could be used like this:
Red Light: STOP the game.
This actually means stop. Restraints are ALL undone, any beating or spanking stops. Game over.
For the more experienced players, the other colours could be used.
Amber Light: Slow down a bit.
The game is still enjoyable, but the intensity is too much. I need a breather. This can either be followed by "Red Light" or "Green Light".
Green Light: Rest brake over, let's carry on.
NEVER EVER MESS WITH THE SAFE-WORD Keep it safe, sane and consensual.