A neg is a pickup method where you attempt to disqualify yourself as a potential seducer: you are trying to show that you are not hitting on her by making a compliment that has a hidden negative element

- "Oh your nost wiggles when you speak" should be said while pretending you really believe that her nose wiggles.

or

- "I like your dress, OMG how they are popular these days" (For a woman to say that her dress is not unique may feel bad)

By doing this you make the girl feel slightly self-conscious while at the same time you send mixed signals and making her thing that you are not that interested to her. You then can move on to flip her 28 attraction switches one by one.