This is a quality asa that has an on/off built in. It is very nice. This allows you to depressurize your gun before unscrewing the tank. Simply unscrew the on/off and fire the gun a few times until it is depressurized. Then unscrew the tank content with the knowledge that no oring will get munched and no newb will go ballistic screaming that you're losing all your hpa. Hpa.
A good idea to also get the prorail from CP as they match and look ubersecks together.