In a reed valve engine, the valve is drawn open by suction as the piston moves upward on the compression stroke. As the piston moves down on the power stroke, the pressure in the crankcase causes the reed valve to close. The fuel-air mixture in the crankcase is then forced past the piston via the transfer/bypass ports in the cylinder. One characteristic of a reed valve is that the engine will run in either direction; an advantage for a "pusher" model but a disadvantage if the engine is finger started, as it may start in the wrong direction. (The Cox engines employed a starting spring which kept fingers free of the propeller and generally ensured correct rotational direction.)