These Valves as the names suggest are operated by external air signal to the Piston Provided inside the valve. This in turn shifts the spool and the cylinder shaft is operated in one direction.

The moment the signal is cut off, the spool comes back by a spring and the cylinder shaft retracts to the original position. The External pilot singnals are generally provided by means of 3 port Roller Valves.

In the type external air signal can be given alternatively to both ends (i.e EP port is provided on both ends). One side air signal will cause the cylinder shaft to move forward and when air signal is given to other side cylinder shaft will retract.