The Airsoft2Go guys explains what HPA is all about in the second season of their Airsoft for Dummies series... "At the beginning of the 2nd season of Airsoft for Dummies, Brush explains to the Dummie Momber what HPA is and what the advantages and disadvantages are. Have fun with the video! Unfortunately we had microphone problems, so Momber and Brush are sometimes very quiet. (next time it will be better)."