When two people throw down on a bowl (if this ever happens) or when two people just want green hit, one person calls corner hit. Then the person who hits it first only hits the corner of the bowl and passes the bowl to the other person who wanted green hit and they hit the other corner.
person 1: Hey, man, I want green hit, too.
person 2: I'll save you corner hit.
When hitting a bowl or a bong, you position the lighter to the side of whatever is holding your weed. you almost suck part of the lighter flame to only a fraction of the bowl the light up just a decent section of weed. Therefore you dont char the shit out of the whole thing. Its courtesy.
The guy before me in rotation didnt corner hit so we pretty much had to empty the whole bowl and pack a new one.