The purge button is the part of a diving regulator that may be depressed manually to force the regulator to deliver air. Normally the part of the second stage covering the diaphragm is made flexible and serves as a purge button, since pressure on the diaphragm opens the valve to release air through the regulator.[1]
If a second stage is removed from a diver's mouth, it will fill with water. One way of clearing the water is to operate the purge button just before the second stage is replaced in the mouth. Another use is for putting air into a delayed surface marker buoy or lifting bag. Any time that the purge button is operated, the diver must be aware of the potential for a freeflow and be ready to deal with it.[2]