Air pressure is just spring rate. More PSI means more ride height/ability to support load.
The QS3 controls compression dampening adjustment. Compression dampening adjustment controls how much restriction the shock oil must go through to allow the shock to move. More dampening results in slower rate of travel for the same load. Lesser dampening allows the shock to act more like an unrestricted spring.
Position 1 - very little dampening/restriction to flow, this results in a very 'soft' feeling shock which is easy to bottom. This is a commonly used setting for riders who prefer a softer more compliant sled.
Position 2 - Medium dampening. Provides significant increase in dampening while still not being super harsh. Allows for reasonable weight transfer but significant resistance to bottoming.
Position 3 - Ski and Front track shocks - Most dampening, greatly increased dampening, This can feel harsh or rigid in many conditions. This is a great option for more aggressive riders when pounding whooped out trails/jumps. This allows for a rider to take much harder impacts without blowing through the suspension and bottoming out.
Position 3 (L) - On the OEM rear QS3L shock. The 3rd position acts as almost a complete stop to oil flow, this causes the rear track shocks to act very rigid and prevent any transfer. This causes minimal ski lift and allows the sled to climb significantly better then the other settings. There is a bypass in case of super hard hits so it will not damage the shock.
