I've been thinking about writing up some of my ideas on safewords for a while – because there seem to be a variety of approaches to them.
To me the absolute foundation to all play is consent – that is what makes me able to look in the mirror every morning and know that while I might be sadistic and enjoy the suffering of my submissives that I'm not an abuser… my slaves, submissives and play partners consent to the activities that occur, even when they might not "enjoy" them in the moment.
The key part of consent of course is the ability to rescind it. Which is of course what a safeword is for, particularly for those who like a roleplay of resistance (which to be honest is not one my kinks). It is to keep you "safe" – to know that you can if required quickly and easily rescind consent and let the Domme know that everything has to stop.
This is the main meaning of a safeword, but I have noticed that different players may use them a little differently – and might use "Mercy" for example a bit like a pause button on a remote control, to get a quick breather or to dial down the intensity a little bit. Some people even like to use a "stop light" system where the bottom can use "green; orange; red" to indicate how much they're enjoying their experience – which is a valuable approach for many, particularly if the Dom/me is less experienced but is not the way that I prefer to play. In fact I prefer to use "fluffy kittens" as a safeword as it is outside of the fantasy realm and something that obviously stops any play.
For me using a safeword is rescinding consent – it means that either the session ends, or (more usually) that we take a little time out and renegotiate. However to be honest this very rarely happens in my dungeon – I try to take a lot of care and enough time with my negotiation beforehand so I have a good understanding of limits before the slave kneels and offers himself up to me. Plus I'm very good at reading when someone is nearing the edge of their endurance and backing off just as they're wondering if they will need to use the safeword…
There is of course another side – it's possible to communicate during play without using the safeword and taking back control… To give me as the Domme all the information I require to torment and control you. Say for example the submissive is having trouble with a sore back in a bondage position – just tell me – everyone's body can react to positions differently and I'm happy to adjust add a pillow under the knees or change something if there's a problem. Another instance is being disciplined with a leather strap, instead of saying "mercy" or "red" and stopping play when they feel that the pain is too intense, they can plead with me "Please Mistress I don't think I can take any more" – that still gives me the power and control to decide if I will take pity on them… Perhaps I will whisper in their ear "surely you can take two more, just to make me happy" – or I will laugh at how they've bitten off more than they could chew and change the play to something else, or say "too bad, I'm not done yet" – but if the latter there will still be a change in tempo and plenty of checking to see how they are handling the intensity. The safeword is still there – but it's not pulled out as a way for the submissive to control the session.
Safewords are important – but they mean "Stop everything and check in". Both the Domme and the submissive are pulled out of their headspace, and so they should be as it means something has gone wrong… Never be afraid to use them, you always have the right to rescind consent – but also consider other avenues of communication before pulling the plug.