Here's the setup: Valve #1 dumps to the open air. Valve #2 is connected to the main air chamber. Both valves start out closed. Once the chamber is charged, valve #1 fires the gun. The trick is timing it so that just before the spud clears the barrel, #1 closes and #2 opens. This repressurizes the back side of the piston, which then closes.
The point of this is to cut down on reload time by conserving air that does not directly act on the projectile. By keeping it in the chamber, less gas needs to be added for the next shot. Theoretically, with a regulated CO2 system, one could shorten the fill-time enough to allow for semi or even fully-automatic capabilities.
