popping your collar is a gesture meaning that you a pimp, E-40 started popin his collar first. you do this when your dancing, takin a picture or whenever you feelin fly. you could also cogratulate a playa by poppin his collar, like when he's dancin with 2 dimez.
your walking by a homie at the club and you see him dancin with a tight breezy, so you pop his collar thus congratulatin a playa for his skills. you could also say "pop your collar, mayne" while poppin his collar.