Well, I think that many of us has experimented with safe-words and other forms of "stop" orders. But my experience is, just like many others, that it really don't work in practice. When the sub is "high" she/he simply forget their own limitations and don't even consider to stop. Else who remember to say "firecar" or what ever. While gagged a ball in a hand can be used. But who remember to hold it getting whipped.
So it's just to know the subs limitations and else simply use your common sense and else notice the subs body language to stop in time.
Else it's very important, to know all about both your own and the subs limitations on beforehand. All to many relations has been ruined by "But I thought you knew" or "But I thought that you liked that"