|Image courtesy of Wikipedia|
When applied as a pneumatic valve actuator, opposing racks are attached to pistons in a cylinder. When one side is pressurized, the pinion bearing turns in one direction. When air or spring tension is applied to the opposite side, the pinion bearing turns in the other direction, thus allowing rotational motion in both directions.
|Transparent actuator showing opposing pistons (ROTEX)|
|Pinion bearing and rack with piston and seal (ROTEX)|
|Housing which forms outer pneumatic cylinder walls (ROTEX)|
Here is a great article on rack and pinion actuators from Valve Magazine.