What are HPA Tapped / Engines ?

In the previous tutorial, we have explained what is HPA and if you are using an AEG airsoft gun

then you would most likely be using an "HPA Engine" with an "HPA RIG" attached to supply HPA to the airsoft gun

(Depending on the Airsoft gun model some may have direct placement of the HPA Bottle directly onto the stock section of the gun)