The Air Valve (operation)

The air valve is a simple poppet type consisting of a valve head (blue) which seats against the valve body under both spring and internal air pressure providing an airtight seal.  When assembled, the Valve Spring retainer compresses the valve spring holding the valve head in the closed position. The Top Hat is screwed onto the Valve Stem and is secured in position with the small grub screw and the valve stem is a push fit into the valve head for which purpose it is splined on its inner end.

On firing, the hammer transfers its energy through the breech slide to the Top Hat which is screwed tightly onto the valve stem. The force of the hammer strike pushes the valve stem inwards driving the valve off its seat and compressing the valve return spring.
Bob Craske

A quantity of high pressure air immediately escapes past the open valve and enters the hollow valve stem through 2 small circular drillings. The air passes up the valve stem, exiting via the Top Hat and entering the breech where it impinges on the rear of the pellet.

At the same time, the energy imparted to the valve assembly by the Hammer dissipates and the Valve Return Spring reasserts itself assisted by the air pressure remaining in the bottle. The valve snaps shut, preventing the escape of any further air until the next firing cycle.