High pressure air (HPA) systems are a type of pneumatic airsoft weapon that use externally supplied high pressure air

instead of internal gas canisters like the majority of gas-operated airsoft guns.

They work by using a separate high-pressure air tank with an air regulator (in which can control the output level of the air) that is connected to

1) a GBB (Gas Blow Black) airsoft gun , via a hose to the gas magazine area of the airsoft gun

OR

2) a AEG (Automatic Electric Gun) airsoft gun , which is connected with a pneumatic motor inside the gun (called an "HPA engine") 

at where the gearbox would be in a normal electric airsoft gun.