How A QEV Works
Quick Exhaust Valves (QEVs) are very common these days on many different paintball guns including Autocockers, Typhoons, Strokers, Ions, Intimidators, A5s, and more. They are very helpful when there is a long air line between the ram or bolt and the control valve (4-way, solenoid, etc.). By allowing the air to vent right at the ram or bolt rather than channeling the air back through the hose the mechanism is able to cycle faster. Here's how it works:
When air enters the QEV from the input (right side) the incoming air pushes the rubber seal to the left closing off the exhaust and entering the ram/bolt/whatever.
When the incoming air is cut off, the exhaust air pushes the rubber seal to the right closing off the input hole and vents the air to the atmosphere.
The QEVs in the picture are from Pneumadyne but all QEVs work by a similar principle.
Note: the QEVs in the picture are rather beat up and I milled one of the parts. Just ignore that and focus on the internals.