From Spud Wiki
Venting is the action of cleaning out combustion products and introducing fresh air to the chamber of a combustion or hybrid cannon. This is necessary to ensure that there is enough oxygen present to sustain the combustion of the fuel.
Venting can be done by simply opening the chamber by means of a threaded plug or cap, a large ball valve, or a check valve; a chamber fan is often used to help exchange the gases. Other venting methods include using a bike pump, compressor or mattress inflator to inject air into the chamber. Pressurized air from a scuba or HPA paintball tank can also be used, but with a pressure regulator.