The carb works like this: it is a hole placed in the side of a bong that you are supposed to cover up with your finger as you are filling the chamber with smoke. Then you release your finger from it, which lets a lot of air directly into the chamber, causing all the smoke to rush into your lungs at once. This is easier than having to draw air into the chamber through the bowl when you're done with your hit, or leaving a chamberful of smoke to get stale.