Sounds like bullshit straight out of that cancerous "Pick-Up Artist" scene tbh.
That's exactly where it does come from, and don't Google for any examples unless you really need to have your opinion of humanity lowered considerably.
Funny thing is, it kind of worked for those guys, hehe. ( ͡~ ͜ʖ ͡°)
The theory of a "neg" is, iirc, that you should hint or suggest to the girl that you have a negative idea of her ⎯ that she's uggs, or dumb, or has a bad personality, or something. Then the girl will desperately try to show you that she's not like that! Then you sit down and relax as she makes every effort to prove herself to you...... and, after a while, you tell her "Ok, you win, you've persuaded me, let's go on a date!" and she's like "Yay! I won!" and doesn't even realize that you have manipulated her.
So you see, you could say that Nanaki totally fell for the neg, as she was furiously trying to make Kanade look cute just to "show those assholes" that they were wrong! If she succeeded, one assumes that the guys would say something like "Yeah, ok, your efforts have paid off, we're convinced, you two can hang out with us!" to which Nanaki would go "YES! I did it! Wait, what???"
(Hmm... come to think of it, it still can happen in ch8...)