This adapter screws onto a standard paintball co2 cylinder (or paintball HPA tank) and adds a shut off valve which you can conveniently use to turn the flow of gas on or off. Great option if you want to leave your cylinder attached to your paintball gun. Added benefit of this adapter is that you dont need a separate shut off valve for each gas cylinder. Can also be used as an "extender" if you find your paintball gas cylinder is too short when shouldering/aiming.