It blows off the pressure inside the discharge pipe when the compressor stops.(There is another type of unloader valve, which can be found at the inlet of rotary screw compressors, also called inlet valve or loading valve). On smaller units, this valve is often mounted on or inside the pressure switch. When the pressure switches off the compressor, the unloader valve is also actuated. The check valve is often located right at the point where the compressor discharge pipe is connected to the storage tank. From this check valve, a small tube or pipe runs to the unloader valve on the pressure switch. Unloading valves is controlled by air pressure from a small pilot valve. Atlas Copco Unloading valve kit part numbers 2901000201, 2902016100, 2901162200, 2901146300, 2901029801.