When you pop your collar, you reach up to the shoulder of the correspoding arm or across the chest to the opposite shoulder, and, using your index finger and your thumb, you pull up on the shirt and let it "pop" down, hence the phrase. It is perfectly acceptable to pop your collar whenever you have accomplished a great, admirable feat, such as busting a sweet dance move, pwning the entire opposing team by yourself, or to establish your extremely high playa factor.

When you eat dirt for the first time and leave your family to build a shack in the woods

To flip your collar up on your polo shirt. Very common practice for guys in fraternities and other college-age preppy types. The guy who does this is often referred to as "that guy."

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.

When one pops the collar up on their shirt(s). Mulitple shirts can be popped at one time. This practice is mostly done by somebody considered "preppy."

AN ACT OF FLAMBOYANCY. as the FONZ from happy days

Chill!