I have never safeworded out of a session, but I have called red when things have become too much for me (including when I have felt myself start to pass out).

Sometimes the reason is obvious as the Domme is specifically pushing me to see exactly how much I can take (especially in the case of corporal) and in those cases the Domme usually just stops and moves on. Sometimes the reason is not so obvious and in those cases, she stops and asks what is going on then corrects the issue.