A Neg is a technique that pick-up artists use to get attention (attraction) from pretty women. Those women generally get all kinds of positive attention from men hoping to 'buy' their attraction with compliments.
To be different, they sort-of-insult the woman, to appear independent and strong and to break down her initial assumption that he is just another guy who will be like the rest.
In short, you playfully point out that she is not perfect and that you see that. A few examples would be along the lines of:
+ "You have a very pretty face, but you should use less makeup. Makeup should be used to enhance a woman’s beauty, not hide it." or You're a really pretty girl, but what else have you got besides that?
There is all kind of theorycrafting on why this works (And why it apparently only works on really pretty women or women who think they are but aren't that pretty.)
But I keep wondering if the whole point of this isn't just to shake someone out of social roles. That for some reason, showing that you can see past the mask and that you accept it, you allow the other to remove it and become more genuine with you.
It would be funny if a bunch of guys wearing silly hats and boas, who insult women to seduce them and to sleep with them, are somehow making the world a more honest place.