The purge button is the part of a diving regulator

Diving regulator

A diving regulator is a pressure regulator used in scuba or surface supplied diving equipment that reduces pressurized breathing gas to ambient pressure and delivers it to the diver. The gas may be air or one of a variety of specially blended breathing gases...

 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.

If a second stage is removed from a diver's

Scuba diving

Scuba diving is a form of underwater diving in which a diver uses a scuba set to breathe underwater....

 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

Lifting bag

A lifting bag is an item of diving equipment consisting of a robust and air-tight bag with straps, which is used to lift heavy objects underwater by means of the bag's buoyancy...

. Any time that the purge button is operated, the diver must be aware of the potential for a freeflow

Freeflow

In scuba diving, a freeflow occurs when the diving regulator continues to supply air instead of cutting off the supply when the diver is not breathing in...

and be ready to deal with it.