When my PB installed my FG pool, I think I remember him installing a hydrostatic value, but not sure. When I read about this now, it looks like it is something normally attached to the main drain and is automatic. What the PB put in was a 3rd value near my pump (the first valve is the skimmer and the second valve is the main drain). He said the third valve could be used to pump out water outside the pool (underground) should I ever have to. That valve has always been closed and I never tried opening it. Next to pool, there is a 9x9 cover that goes down the depth of the pool, but is never filled with anything. This was part of that 3rd valve install.
Is what I am describing anything to do with a hydrostatic value?
The PB is no longer in business.