Mini Paintball/Airsoft m203
It was kind of hard to understand that, crowley, so I'll elaborate. With spudguns, it's so your gun doesn't reach unsafe pressures. There is a 125 pound spring inside the valve, so if your gun gets to 130 psi, it's push the valve open until it gets down to 125 psi or lower. Then the pop-off valve closes and shuts off flow again. You can also pull the ring to manualy realease pressure. He uses the ring to release pressure behind the diaphragm and shoot.
EDIT: Didn't see page 2.
EDIT: Didn't see page 2.