The ABV-1 Ambient Breathing Valve (910271-000) allows the user to breath ambient air on the surface. This helps conserve air while you wait for your dive buddy or while on the surface.
The valve is easy to use, simply turn the valve counterclockwise one turn. Outside air will pass through the one way valve. It clears the lens then enters the (2) one way valves into the oral nasal cavity. You exhale through the 2nd stage regulator. When ready to dive, close the valve and you are on the way.