I'm no pua expert and it seems like a way to scam virgins.
But negging is a way of making yourself stand out
The assumption is that, with a hot girl, she'll be used to every guy coming up and telling her how pretty she is and being super nice obviously to try and fuck her. She'll be numb to it and used to the attention.
If you neg her, the idea is that suddenly you're way more interesting than every other guy and must have much higher value
It makes sense if you think about. If you spend your days surrounded by 10/10s and then go to your loser friend's party where all the girls are 5/10s, you're hardly gonna be impressed
But in that friend circle, a 6/10 would be the equivalent of a 10/10
By not giving a fuck and calling her out on her bullshit, it becomes evident that you're just trying to get in her pants
But I think there's a difference between treating a hot girl as if she's not hot, and purposely acting like a dick to her for attention
but both are strategies so...