Usually when one pinches the sides of your collar, usually on a polo or dress-shirt. You pinch the sides, and pull them up in self appraisal. It's a way of saying to the world, "look at how great I am". Almost exclusively male.
For preppy teenaged boys and girls;
Usually when the boy or girl unfolds the collar of a polo so that it stands straight up around the neck. It's supposed to be 'cool', but you can sometimes come off looking like Batman.
For preps; "Nick was popping his collar in class today. At first I thought his ears were cold, but then I realized he thought it was 'cool'."