For me a safeword is very useful because in a heavy scene I tend to go pretty non-verbal. Explaining that something is wrong would be totally beyond me. A monosyllabic safeword can still take a while to summon up, on the few times I've used mine. It tends to mean more than just 'something's wrong', it's 'something's wrong and you need to bring me back to earth enough to be able to tell you what it is.'

And the other aspect is that I want to be able to say 'no, I don't want that' without being taken seriously. While playing without a safeword and with a previously bruised ass I was asked early in the scene if I wanted him to cane my ass. I didn't want to answer because it wasn't as easy as yes or no. I was feeling somewhat belligerent and not really into the pain. There was no way I could convince myself to say yes. But I didn't really mean the no. Yes would have meant more whacking on my ass than I would have enjoyed, but I didn't want it neglected entirely... but I settled on 'no', and he listened. Which is a good thing when no still means no but I wished that it hadn't, and my 'no' required less consideration. Because being pushed into what I don't really want to do is hot