I always want people to know they can stop the scene if they need to. I ask them if they have a safeword. Most don't, so I assign one. I make it clear that I want them to use it if something is really wrong and they want me to stop the scene entirely. (I have never had anyone safeword out.)

I tend to like to use the stoplight method as a way of getting to know someone's subjective experience. It's a good way to simplify communication without breaking the mood of the scene. Though I'm good at intuiting and reading where people are at, once in a while there's a person who is more stoic and difficult to place.

So, I tell them that if they feel I'm close to crossing their threshold from hot to not to say "yellow" and if I hit it, but they don't want to stop the scene, to say "red". This allows the client to feel that they have some control over the scene if they need it to keep enjoying themselves.

While I've never had any say red either, and I usually know when to stop before a yellow, once in a while someone will say it. Most of the time they tell me they said it because they were afraid. Then I resume with them feeling more comfortable about my being in control but still respecting the limits of their submission.

In some cases, people don't want to use them at all and take whatever I give them. Of course, that's absolutely fine! But some people are very nervous (usually new players in the first or second sessions) and this helps them relax into the scene better.