Our church pool is drained each year and when it's empty, groundwater comes up through the hydrostatic until it finds its own level. When the pool is full, the weight of the water and the water pressure is enough to keep it closed. The hydrostatic valve is located beneath the drain grate in the deep end of your pool. You have to remove the grate to see it. The valve on the church pool is a 1 1/2" pipe with a cap that is attached with a hinge on one side. There is an O ring below the cap and it is spring-loaded so that the cap is closed when something is holding it down (ie. water/pressure). During the winter, we put a large rock on the valve to slow down the influx of water. As I understand it, almost all pools built in the last 30 years have them, and they are probably required by local building codes in most areas.
Hal