I too have never used a safeword. I think, as was mentioned before, this is possible if the dominant is extremely in tune with their sub. We have a safe signal (I'm usually gagged) of 3 short moans in a row. This stands out from the usual noises because it is so deliberate. Still, I haven't yet had occasion to use it, after about 5 years of pretty heavy play.

The local play parties here insist on the yellow, red system and that a gagged sub holds keys or something similar to drop. My problem is that often if I'm experiencing pain or euphoria I'll flex my hands alot and have a hard time holding onto anything. You have to play by the safety rules though.