Thought I'd add something interesting on how the Huben closing valve works, and it is in contrast to the Sidewinder, which despite their similarities does not behave the same way . . .
On the Huben, for a given functional setting of the closing valve (meaning something usable between either fully open of fully closed) the amount of air released on the shot is a direct function of the resistance that the projectile provides to the air flow. This is very different than a normal valve.
I have mine set to shoot 18.1 grain pellets at about 920 fps. At the same setting, it will shoot 15.9 grain Hades pellets right about the same speed - indicating it is using less air per shot with the lighter pellets. And if I dry fire it with no pellets in the mag, it functions fine but releases very little air at all - much less than on a full shot. And at the other extreme is when the gun jams, which it has done when I tried to shoot slugs that were loose in the mag - with the slug blocking the airflow path, the closing valve does not close for a long time, and the gun can dump the reservoir on one shot. I understand how this happens as the closing valve requires a drop in air pressure in order to close - drop the pressure faster and the valve closes faster and vice versa.
The Sidewinder does not behave this way - when dry fired it lets out pretty much a full blast of air as it would with a pellet in the mag, and lighter pellets go much faster at a given setting. I have not had it apart, nor have I seen any kind of parts diagram, so I don't know what the difference is but it sure behaves differently when shot.
One of the other great things about this design is how the magazine advances - the actuator is basically a sealed piston. When the shot happens and pressure drops in the firing chamber, a spring overcomes the lower pressure pushing the piston forward, and then when the chamber represurizes after the shot, the piston returns against the spring and that advances the magazine via an internal gear. No wasted air . . . brilliant!
