NTR is about stealing, mental abuse/bullying/domination depend on the character, there are stories where the antagonist simply seduce the girl away, by showing he is better than the boyfriend, and he don't give a fuck about the boyfriend, he just want the girl, but yeah, most story are about bullying, it's the mainstream, and people believe that NTR is about that, but it's just stealing, heck I still remember a manga where a guy could turn the spell of an enemy into his own spell, and he litteraly called that ability NTR