The Legal, the Emotional and the Medical Side

1. The Legal Situation

Like I said before, bondage should only happen between two people both consenting to this experience voluntarily and knowing what they are doing. And the safety rules are a must of course. The consent of your partner can be given verbal or non-verbal (for example by putting out the wrists for tying up), but it should always be explicit. If you are not sure whether your partner consents or not, just ask him. Without consent a bondage sessions legally means unlawful detention. If furthermore you have sex with your partner during such a session it's sexual duress or rape. And if some damage is done like bruises or nerve irritation it's bodily harm as well.

Safety Codes
Bondage is a joyful game for both. To keep in a kind of emotional contact all the time the active part has to know how to communicate with the passive part, and the victim muct have a chance to communicate his or her state via unambiguous signals.
In the non-commercial SM subculture there is a whole range of codes and safety words. You should know them even if you only like bondage and don't want anything to do with the scene.
Especially when words like "please don't" or "no" are part of the game, it's all the more important that there are some signs which can't be misinterpreted.
On the one hand there are stop codes; they can be a certain word as well as a certain signal. In the german subculture the most often used word is "mayday"; but other words like "marmalade" or "circus" are possible as well. The main thing is that it is a word that doesn't get over your lips by accident because it's unusual somehow; it should be easy enough to pronounce as well, though. If you broaden your bondage game so that the passive part is gagged there should be a certain sign meaning the game has to stop at once.
Because a stop code is something the passive part has to use actively while there are some situation where he or she is no longer able to do this (during an emotional crash for example or because he/she belongs to the kind of person who just can't say no) confirmation codes have been developed. This can be a kiss returned, an exchange of glances, or the answer yes to the question whether everything is all right.

Then there is a third category of safe words for the fine adjustment. To this category we count the stoplight code: Green for more, go on; yellow for less, go on a bit softer; red for stop, I need a short break. There can be individual alternatives to this.
In any case you should agree on the kind and meaning of the safe words beforehand to avoid misunderstanding.

From: Das Bondage-Handbuch, Matthias T. J. Grimme, Charon Verlag 1999