Just a brief look at the variable volume chamber. This is adjusted by a screw at the rear of the Prop Shaft: -
The Prop Shaft is hollow, and stores additional air, over and above what is in the firing chamber. Adjusting the screw moves a piston up or down the hollow bore of the Prop Shaft, adjusting the volume that is available for air to be stored in. This gives you a small degree of adjustment over the Geo's operating pressure.
posted by Uziel Gal in the 4th thread called stickies, if you look at these threads they are full of good advice and mention many ways to adjust you marker. But everyone likes there's a certain way so only by testing will you get to know your marker and what its capable of doing for you.