The fact that one speaks out beforehand has probably always happened in groups of sadomasochists; the formulation as a principle was first carried out in the USA. There we speak of negotiation. A full consultation will address at least the following points:
Bottom's preferences.
The classic theme of collusion. The bottom lets the top know which practices, role plays, etc. he particularly likes or, on the contrary, completely rejects. It is not enough to use keywords. By humiliation, the bottom can mean that he is to be raised into an actively serving slave, but the top may imagine the bottom as a passive "object". The bottom does not necessarily have to speak for this part of the agreement. Sometimes it's enough for the top to imagine different scenarios and pay attention to which ones particularly appeal to the bottom. The more direct the communication, the more likely the game will match the fantasies. There are also written forms of agreement, either in the form of the bottom making a kind of request to the top or as a questionnaire on which a scale from "always desired" to "definitely not" is set out for each practice which is filled by the bottom.
Boundaries
Since very few tops see themselves as fulfilling their bottom's wishes, they will not follow the bottom's catalog of preferences, but will instead try to implement their own ideas as much as possible. To do this, it must be clarified roughly where the bottom's boundaries lie and how much power he wants to cede to the top. The demarcation will be somewhere between the poles of “only known practices, with known severity” and “Top should just do what he wants”.
Medical questions
Often overlooked, but fundamental to security. Here the bottom informs the top of possible problems in advance: a chronically stuffy nose (no gags), damaged intervertebral discs (no hogties), claustrophobia (no hoods), blood circulation problems or tendencies to thrombosis (no kneeling for long periods), bleeding tendencies (no cutting games), whether the bottom has already lost consciousness during orgasm (see emergencies), etc. In a broader sense, it can be found out here whether the bottom is physically capable of certain practices, e.g. whether his elbows are comfortable for being tied behind his back let lead together. In extreme cases, certain practices will have to be completely excluded through the agreement. It is therefore questionable whether one would even want to restrain someone with a strong tendency to epileptic seizures.
Determination of the safe word
The safe word is discussed in more detail there and also under Security. Before a game, it must be clear to all players whether a safe word is being used and what it is called. In case of doubt, Mayday is probably the most common one. In addition, slow words can also be set here, which give the bottom an opportunity to verbally control the intensity during the game. The importance of a detailed agreement can be shown using an example. Impact games can be used to create pain (see SM) or less intensively to humiliate the bottom (see DS). If the top doesn't know which direction the bottom's preference is, the only way he can find out which is better received is by trying it out. Some sadomasochists have reservations about the agreement. One of the most common is that the process would destroy spontaneity because the element of surprise would be lost. In fact, there isn't much to counteract this, especially since "extradition to the unknown" is part of many fantasies. A conflict can arise between the need for security and fantasies, which must be resolved by those involved.
Denying the bottom's desire to set boundaries is at least bad form - it's more like emotional coercion. In any case, wanting to say a few words about your preferences and limitations has nothing to do with a lack of commitment. If in doubt, you have to accept the irreconcilable differences and look for other playing partners. A one-off, thorough agreement can perhaps take away some of the atmosphere from the following game, but it provides a solid basis for all subsequent games that can be built on for years to come. The agreement can also take place some time in advance and at least the first part can be packaged in a different form as "I want..." - a story, a reference to a film scene, a scene that both of you watched, also works. Over time, some people want and also need a new agreement (renegotiation) in which the partners bring their preferences up to date - others get to know each other so well that the need for further agreements disappears.