So i don't really have a safeword since i made the transition from sub to slave. What this means for me though is that my Daddy will respect my limits and will respect if i call out red (my old safeword system was the color system), however he chooses whether or not to stop. He has never pushed me too far, and often i underestimate just how far i can go, so it's nice to know that he can read me really well and make sure i am taken care of, that i can be pushed to the breaking point and at the same time kept very safe. It also helps that all my limits are things he is not even interested in!
We are still learning as we go and finding what works and does not, but the way we feel is that if i am wanting to give myself over completely, a safeword is my holding onto some control. It is not my choice to stop, rather his, and i always trust that he will never harm me (although hurt me is another story hehe).