|The valve has a number of components:
A plastic stem with a hole in it. The product passes out through this hole when the valve is opened. The valve is kept closed by ...
... a rubber ring that slips over the stem. When closed, this seals the hole in the stem. When the actuator is pushed down, the ring is bent upwards and uncovers the hole - letting the product through.
The housing which supports the rubber ring and forces it upwards when the actuator is pushed down.
A cup that holds together the valve components and the can.
A spring that pushes the stem back up when the actuator is released; the rubber ring then re-seals the hole in the stem.
When the valve is assembled, the rubber ring sits on the housing. If the stem is pushed down by the actuator, the rubber ring is bent upwards and reveals the hole. Product then passes up the inside of the stem, through the hole and into the cup. This directs it up through the actuator.