How does ft become forced?

Let's assume that if a guy asks for or wants something that it is not forced. How do some regular ft sessions becomed forced, when a guy is backing out or wants to say no? Obviously dome dommes will insist and force the guy or enforce some agreement and some dommes won't. How do you know whom to force and whom not, if any?

I think this can never happen and it is illegal and possibly a crime.

You might have read a story where a mistresses forces a guy to eat her poop and he begs to stop but she doesn't and continues to feed him until she is done. Or when a guy goes to see a mistress for a quick pre-arranged FT session but she locks him up and tells him that he will be used by her and her friends multiple times a day for a month. That's just fiction. In reality you always have a safe word and you can quit the session whenever you like.

However if before using the safe word she seduces you and plays psychological games with your mind and makes you agree to do something that you had not agreed to do at first, it is no longer forced because you have agreed to do it.

I think another reason why dominas never force you to do something that you haven't consented to do is because they know if they ever really wanted to use you for FT, they can play with your mind over multiple sessions so that one day you beg to be fed.

All said I have recently realized I know very little about BDSM so it is better to hear an experienced forum member's views. 2020 is getting more and more unreal for me!