Like most parts on a BCD, OV-185 scuba over pressure valve is an incredibly simple device. The valve is normally closed and will not open unless the air pressure inside the shelter is slightly greater than the outside air pressure. Scuba over Pressure Valves have a ball-and-string assembly attached called a “dump valve.” Pulling on this ball-and-string will release air from the BCD through the relief valve. If the pressure increases, the spring won’t be strong enough to hold the plug in, and the BCD will release air. The opening of a scuba over pressure relief valve will give a pressure build up in the piping system downstream of the relief valve. Also ok to manually override the valve by pulling the plug yourself, which is what you are doing when you pull the cord?The simplicity of this device helps make BCD inspections a fairly easy process, especially when compared to overhauling a regulator. On back inflation BCDs, the valves are also located on the lower portion of the BCD, but may face either the tank or the diver. To use a rear deflation valve, a diver must be in a horizontal swimming position. He simply sticks his rear end up a bit and pulls the over pressure valve release. The simplicity of this device helps make BCD inspections a fairly easy process, especially when compared to overhauling a regulator. OV-185 scuba over pressure valve. Suits all current Aquatec diving BCDs and wings. Coarse thread