This is the Apeks Freeflow Control Device Features: Isolates free-flowing second stages Guards against unexpected air loss Improves diver safety Gives total control of a frozen second stage Fits any second stage with standard hose fitting Regulators not designed for cold water use are especially prone to freezing. Our simple yet extremely effective Freeflow Control Device (FCD) contributes a significant advance in diver safety. In the event of second stage freezing, free-flow can cause catastrophic loss of air. With the FCD fitted to your regulator, a simple sliding action allows the diver to isolate the free-flowing second stage and continue the dive using a secondary air source or alternatively, the diver can manually control the air flow allowing a safe ascent to the surface.