A carb, or "rushole, shotgun" is meant so that you can have a source of pure air when you're hitting it. Cover it when you start to spark it, when you throat starts burning a bit, uncover it, recover it, uncover it etc. until you've taken a satisfactory hit. The point of a carb is just so you have more control over the size of your hits, and decreases the chance that you're going to cough.