Those two things enable Submissive to avoid mental and physical harm during session.

Safeword
There are BDSM practitioners who refuse to use the safeword. In their thinking using a safeword means an incomplete submission. Of course there is a grain of truth. Perhaps it is comparable to a tightrope walk, which is more thrilling without a safety bond than with one. But who is that insane to gamble with the own life just to get the kick?

The permission of a safeword is a guarantee for both sides, that the submissive consents every action. If the submissive feels a limit and is not able to bear the session - from physical, psychic or other reasons - the use of the safeword allows to abandon the session. The termination of the session is mandatory for the top, as there is no existing consent about the exemption from punishment for the action any more. Besides the legal aspects, the top has the advantage of reduced responsibility for his/her submissive.

By saying the safeword, the submissive becomes the leader of the game. It should only be used if the personal edge for suffering is reached. If the safeword is used just to evade an unpleasant punishment, the balance between dominant and submissive part is destroyed. The top would not really be allowed to decide and the game loses its excitement for both partners.

Choose the safeword with caution. It should be a word, that is usually not used during session. Remember: „Ouch! is not a safeword“. International safeword and at parties „mayday“ is used. I use myself „mercy“.

Be aware: the agreement about the safeword means acting in consent even if submissive says „no“ or „please no“ or is crying. The top may respond to it, but it´s not a must be. If top and bottom wouldn´t have agreed to a safeword, the dominant person would have to ask every time whether submissive is ok. Otherwise it would be a risk of harming to one or both of them. The session would be shattered.

The word should be short and easy to be pronounced. Most people suggest a word that has no connection to BDSM. Psychologically this eases the use of the word if necessary. Submissive is double protected by the word: as a submissive but also as normal person/human. I know people who use for those reasons words like: „Newton“, „Pie Plant“ or „Fir“. For your fantasy is the sky the limit.

Sub´s not always able or allowed to talk, especially if he/she is gaged. Depending on the reason for her/his verbal silence, there are different possibilities for a sign. For example could be signs: nodding ones head three times, knocking or snapping with the finger.

Even if Sub is gaged, he/she can get an object in her/his hand, which he/she can drop on the floor or on Dom´s foot. The disadvantage may be that Sub has to concentrate to keep the object in her/his hand. This might be frustrating as it could hinder to fall into subspace.

It´s surely not a shame to use the safeword. If said once, Top has the certainty, that it is allowed to be said. A safeword should not be concealed out of proud or fear. Body and soul of bottom may react very differently. Those stimuli that were just now enjoyable, may be too much some time later. Top is never able to feel or know exactly. But if Top has enough sensitivity and experience, he/she is sometimes able to anticipate the edge.

If the safeword is used more often, it may be, partners are incompatible. They should think over their relationship. Top should be aware: the safeword is just an additional safety, but no safety means 100% protection. It may happen that bottom is not able to use the safeword or sign (maybe he/she is trapped in his/her own world). Top has to watch out the responses of the bottom. The more well attuned to each other, the less energy is used to watch out. The less used to one each other top and bottom are, the more top has to make sure of the responses.

Speed limitNone more so than for beginners and partners not used to each other a direct feedback could be useful. The term Speed limit summarizes different codes. The most established on is the traffic-light-code. It´s just one out of a lot.

Others are: Okay-code or Slow-word. Okay-code means eye periodic contact and if answered, everything is ok. Slow-word means to scale down.

Every code describes the same: Top gets a direct feedback, which is a sign or a word. The traffic-lights are most often used. If bottom says „green“, it means „ok“, „yellow“ is an alert-sign and means „i am near the edge“, „red“ means „stop it, it´s too much“.

What exactly is meant by using colors should be cleared to avoid a misunderstanding. Especially with using the Slow-word, bottom determines the play. Also with using the traffic-light-code, bottom is allowed to determine the play, if „green“ means „I need more“, „yellow“ means „continue“ and „red“ means „lesser“.

I think, that those systems do not reflect the principles of domination and submissive behavior (keyword wishlist-sub). But of course, everyone who lives/plays BDSM uses and enjoys his/her own system of safety.