Usually constructed out of a circular fibre disk or rectangular wooden board, in either case faced with soft leather to act as a sealing medium, and connected to a motor by a threaded phosphor-bronze wire.
The valve is used to block off a boring (wind-way in a chest) and open the boring when the motor it is attached to inflates or collapses. Very often, two valves are mounted on a single wire in order to provide a change-over effect (charge / exhaust); the air supply controlled by the valve is then used to drive a larger motor connected to the boring, eg to open a pallet or drive a percussion beater.
This type of arrangement is, therefore, a pneumatic amplifier.