The Air Pressure System operates like this; one pumps air into the resovour. The Pressure in the resovour increases. One would then pull the trigger which would open the valve inbetween the nozzle and resovour, creating a way for pressure to be released via the nozzle. The pressure brings with it water and air from inside the resovour.
It has been noted that an Air Pressure gun seems to get augmented abilities if put in a freezer before filling it and using it.
Variations on the Air Pressure System
Multi-Tank Varient
This Varient is basicly a Constant Pressure System without a rubber CPS chamber, but with the extra plastic chamber. And it just isn't CPS without that rubber chamber. It is just way too much like the Air Pressure System. So that is why it is classfied as an Air Pressure Varient. The Multi-Tank Varient is just about as common as the Air Pressure System itself.