Safe words are used to keep you safe. They should be used when you need to be let out of bondage because you can't take it. They should be used when there is no way you can take any more of whatever abuse I am piling on you.
Safe words should not be used, in My opinion, to direct the dominant. If I am playing with a beginner to bdsm, you may be tied up and very afraid of what is going to happen to you, however I am not an idiot or crazy person. I am a sadist, top, and caring dominant who is only interested in completely consensual play that is enjoyable for all involved. In other words, I am not going to hook a beginner up to My electrotorture machinery and dial it all the way up. I am going to slowly but surely figure out the way your body and mind work, and keep you at your personal edge. I am aware for different people, beginner or not, that is a different place entirely.
For some it doesn't even involve pain.
I would really be an idiot if I just took every person who entered My dungeon and blasted them with pain for no reason. I am aware that some "dommes" do this, so perhaps some are confused.
And you may watch videos where a man is taken this way... guess what? That man usually consented to that and the truth he is in that body, in his head, completely in heaven, with all that happening to him that you can't fathom.