A two-way check valve allows compressed air from two different sources to flow through the same air line. The valve will select the source with the higher pressure. In this case, the foot valve and hand valve are connected to two-way check valves that in turn lead to the tractor protection valve, and from there to the control (service) line. This arrangement allows you to send compressed air to the trailer brakes by activating either the hand valve or the foot valve. (The foot valve will also send compressed air to the tractor brakes.)