Purge button

The purge button is the part of a diving regulator that may be depressed manually to force the regulator to deliver air. This is either a separate part mounted in the front cover or the cover may be made flexible and serves as the purge button. Depressing the purge button presses against the diapragm directly over the lever of the demand valve, and this movement of the lever opens the valve to release air through the regulator.