I've briefly discussed this topic before, in my post about contracts but I wanted to discuss it properly, too. Firstly, because there is no consensus about whether you need to have safe words in the BDSM community and secondly because I want to help people understand what their purpose is so they can decide if they're right for them.
When people tell me that they don't have safe words, it makes me uncomfortable. They often say that they don't need them or that they know each other very well but it just causes me to worry. No matter how well you know a person I think it is important to have safety measures, just in case. Even if you know how much their body can cope with on a normal day, there is no guarantee that it will cope with that the same way each time. Moreover, if you try something new and your partner can't communicate with you, it can cause issues for you both. I would strongly recommend that you have safe words even if you don't use them all the time.
For us, there are quite a few types of "safe word", though most are not spoken. I don't actually use my safe word when we play because I just don't feel comfortable with vocalising something that could completely stop what's going on. Generally speaking, I don't ever want things to come to a halt; I just want a moment of his time to let him know what's wrong. I'm lucky in that Daddy usually has a good idea of what I can and can't take so he always finishes before I get pushed to the edge of my limits. If this changes and I need play to stop immediately I will use my safe word, but until that point I will use gestures or movements instead.
So, what are these safe words, gestures and movements?
Safe words are as they seem; words spoken by the submissive partner to keep them safe in any type of play. The word used can be anything the you have agreed on, as long as all partners know the significance and will stop play immediately to find out what is going on. Generally speaking, it is better to use a word that doesn't usually turn up during a scene or sex so that the dominant partner(s) know exactly what you mean when you say it. Safe words such as "no", "don't" and "stop" tend to be ineffective because in moments of panic people say them without thinking.
A popular system I've seen mentioned around the internet is the traffic light system. This system is very easy to understand and use, but I'll give it a short explanation anyway. When the submissive partner says "RED" that means all play has to stop immediately. Something is often wrong and needs dealing with immediately. When they instead say "YELLOW", this usually means that they are getting to their limit on a certain thing or that they want the dominant/s to slow down. Finally, "GREEN" means that they are happy to continue. It's a great system to use if you play with someone you don't know because of how simple it is and it will alert any onlookers to how you're feeling as well.
However, if you are playing with someone you know very well you can use your own personalised words. In my opinion, this is very intimate (I find a lot of things about BDSM intimate) and makes your scene or play more catered to your personalities. When you play this way, you can use in-jokes or whatever you want. When we decided our safe word, it was extremely easy and we barely had to think about it.
It is important to note that some clubs will have their own safe word systems in place to protect their customers. If you use a different safe word or are likely to use the traffic light words as part of your scene then it is best to inform a DM (Dungeon Master) beforehand so that people don't get the wrong idea. Anyone who is watching you will likely be watching out for your safety as well!
Safe gestures are much like safe words in that they are a signal from the submissive to the dominant to stop or slow down. The difference is that they involve your body rather than your vocals. Safe gestures are usually important to have, because submissive partners don't always have the chance to speak. If you love gags of any kind or are into zentai then this is something you ought to pay attention to. Daddy and I use this type of safety method the most so I recommend it highly.
A common gesture I have heard of is giving the submissive partner a ball to hold. The idea here is that they will drop it when they need things to stop and the sound will alert their dominant/s. Personally I'm not sure about this one: if something feels particularly good or intense it can make you drop things anyway so we don't use it.
The gestures we use are simple to copy, should you wish to.
If my arms are free, I will instead "tap out" like they do in wrestling. Three taps means to stop and see what's going on. Why three? Once or twice can be done accidentally if I thrash around from pain or pleasure, which is why we decided on three.
If I'm face down and getting a spanking, I will cross my calves over one another to form an X. This is particularly useful because most of Daddy's attention is on my ass and lower half, so he notices this signal much faster than anything else.
I find my own gestures much more useful than the ball one, since you don't need any props to do them and have less chance of accidentally safe gesturing. The other thing is that it is easier to see these gestures in motion and they can be seen even if you're in a very loud place. On the other hand, they also work well in places where you don't want to be heard as they can be completely silent if you want them to be.
Now, safe movements are something I hadn't actually seen around until Daddy and I invented them. This sounds very pompous and full of myself but is pretty much the case. We came across this idea when I wondered out loud how I would safe word or gesture if I was bound and gagged. That sort of situation requires a lot of trust on the part of the submissive partner in that the dominant one is expected to know their limits and such without too much effort. The same goes for the reverse as well; the dominant one must trust that the submissive can take what they're dishing out and must constantly look for signs that something is wrong.
I didn't like this very much, if I'm honest. I hated that the responsibility was all on Daddy to magically know what was going on, particularly since we had only really just started experimenting with BDSM and I wanted to make it easier on him. After some discussion on various things we could use, one of us suggested a sharp movement to the side. We discussed it a little more and decided that was perfect for us.
What is good about this method is that it can be used even if you can't see, speak, hear or move. All it really takes is a firm movement to one side or another (for us, I turn sharply to the left) for the other partner to realise what is going on. The key to differentiate it from an involuntary movement is that every part of the body strains one way, including the face for as long as it takes for the other to notice. Since it is so determined, it is easy to spot (for us, at least). What it means can be discussed between you and your partner/s but we use it as a signal to remove whatever is on my face and mouth so that I can speak.
With all of these methods available, I can't envision an occasion where I won't be able to do any of them. I feel as though there will always be a version I can use that will keep both Daddy and myself safe. You need to remember that in a relationship like this, the dominant partner needs just as much protection as the submissive one does. Any blame for a scene going wrong usually falls on the dominant's shoulders even if it isn't their fault. Having safe words, gestures or movements means that both partners share the responsibility evenly. Mistakes can still be made even if you're well-prepared so it is best to minimise the risks as early as possible.
The best way to decide what works for you is to talk it through with your prospective partner/s beforehand and try out each type in a practise session. It isn't always easy to know what someone means just by their verbal description and because some of these require bodily movements it is best to show them what you mean. This will prevent misunderstandings and make your communication a lot better if the moment to use them arises.
Even if you never use the methods you have outlined, it is always better to have them. You can never tell what the future may bring, so it is best to be prepared to keep you and the partners you play with safe.