It’s doing non-consensual things (e.g., rape) that a person has consented to before the fact. Different people do it different ways, there’s usually some kind of safeword, and it typically involves ‘forced’ bondage and sexual acts. Like, I was dating someone for a while that wanted to be abducted off the street, strangled, tied up, and forcibly sodomized. That was their fantasy; they didn’t want to know when it was going to happen, but they consented to it–and said they were consenting to whatever happened during it–beforehand. (It did not happen due to logistics; that’s a very, very high risk scenario, since at least part of it plays out in public.) This person wanted to be used and abused by me for my own sexual gratification. They was much more of a masochist than I am a sadist.

CNC gets risky in other ways, because it’s easy to blow right past limits without realizing it, and get into sexual assault territory; part of the ‘game’ is that it’s ‘non-consensual’, so it might be difficult to know when it’s become non-consensually non-consensual.