Took one apart - Curiosity killed the cat!!! the thing is actually a twin with two cylinders and two sets of valving - the top is a trick with porting as the middle port is the outlet and the side one is an inlet that ports around and under the pistons. The intake valves are on the tops of the plastic pistons which are captured in a loose fitting membrane seal. A small angular shaft eccentric makes the piston assembly oscillate as an opposed pair. There is a valve in each cylinder and they port to the outlet nipple. Actually a neat little design and they are very efficient for power input to air delivery.







here is a video of it in action..