Some bongs have a small hole, called a carburetor or carb, choke, clutch, shotty, shotgun-hole, rush-hole, air-hole or even just finger-hole, in the chamber across from the stem. This is simply a valve that the user places his/her finger over when inhaling, allowing the chamber to fill up with smoke. The user releases the carburetor, and then he/she does no longer having to pull the smoke through the downpipe and water so it is easier or more relaxing to smoke. The fresh air mixes with the smoke and forces the smoke into the user's lungs more rapidly than could be achieved through normal inhalation. Inhaling a lung full of smoke is known as a hit.

Carburetor hole bongs use a hole to release the vacuum which allows air into the chamber. This has the effect of clearing the chamber of the smoke which is inhaled into the lungs. This method prevents the need to fill the chamber with carbon dioxide to clear the smoke before it becomes stale. They are easy to construct and often used in home-made bongs.

From Wikipedia