The Ocean Technology Systems (OTS) ABV-2 Ambient Breathing Valve with Visor allows the user to breathe ambient air at the surface, conserving your tank air. It is simple to use and its low profile keeps it out of the way. It is simple to use and its low profile keeps it out of the way.To use the device, while on the surface simply rotate the valve counterclockwise. The one way valve will open and allow air into the FFM. The one way valve located on the inside of the full face mask ensures that your exhaled air does not go back out the valve. Your exhaled air is sent out through the second stage regulator ensuring you do not get a CO2 build-up in the full face mask. The valve also is forced closed when diving. The over pressure in the full face mask assures that. If you forget to close the valve before diving, the valve disc will help keep water out. When the valve is closed, it has a low profile. Even when the valve is fully open, the profile changes slightly. When the valve is open the diver breathes normally. He will notice little effort in breathing while he conserves his tank air. The Ocean Technology Systems (OTS) ABV-2 Ambient Breathing Valve with Visor is a must for all Search and Rescue Teams, Military Divers and Divers who spend a lot of the time in the water!