
When connected to a water supply (which can be from either pressure reduced mains or tank) the valve opens to allow water to enter the bottom of the growing container to a pre determined and pre set depth (usually 35mm). The valve then closes and will permit no further water to enter the container until all the original supply has been conveyed from the water chamber to the pot and consequently to the plant. This absorption is achieved by capillary action that naturally occurs in the growing medium. Once the water has been absorbed to the extent that the film of water under the valve has gone, the valve re-opens and another supply of water enters the growing container. This is how the damp and drying out cycle occurs.
from: autopot.com.au