sentences:  In order to expedite the draining of air from the front of the bolt, many users add a quick exhaust valve (QEV) to their marker.The QEV replaces a fitting on the Ion's body and allows the draining air to vent into the body cavity instead of passing all the way back through the solenoid.By allowing the air to vent faster, this allows the bolt to close faster and with more force, which permits lower operating pressure and dwell (the time the solenoid valve is open).