Pilot operate check valves have a small pilot line, shown as a dashed line, that is used to open the check valve and allow flow back through the valve. A common format is the double PO check sandwich plate valve, shown below, that is often used under CETOP directional valves. This allows free flow in both directions when pressure is applied to one side but when the directional valve is closed and no pressure is applied then both check valves close, holding the load in place.