When commercial divers operate at great depths, they often spend prolonged hours in diving bells under pressure breathing a gas mix like hypoxic trimix or similar, that contain the gas helium. Since helium molecules are the second smallest found in nature, the gas is able to work its way inside the watch, around any o-rings or other seals the watch may feature. This isn't a problem as long as the divers stay under pressure, but when they resurface, the helium inside the watch starts to expand quicker than it can escape, leading to an increase in pressure inside the watch. This can cause great damage to the watch, even making the crystal pop off.
To prevent this, Rolex and Doxa S.A. invented the helium escape valve in the 1960s. This is a small, one-way valve, usually featuring a screw-down crown on the side of the watch. When the diver starts to ascend he or she simply unscrews the crown to the full open position, allowing any helium that may have been trapped inside the watch housing to escape. Usually used in a dry environment (inside a diving bell or in a saturation chamber), the fact that the valve is one-way also means that it can be opened while the watch (and diver) is submerged.
Helium release valves are primarily found on diving watches featuring depth resistance greater than 300 m (1000 ft). Models that feature a helium release valve include most of the Omega Seamaster series, Rolex Sea Dweller, some dive watches from the Citizen Watch Co., Ltd and select Doxa Models.