Consider the flap at the bottom of a filled toilet tank.
It snugly covers a hole in the tank bottom. The hole leads into the toilet
bowl.
The flap is held in place by the tank water pressure on its upper surface,
preventing water flow from the tank into the bowl. The flap's underside is
in air.
When the (pivoted) flap is raised by an added, momentary upward mechanical
force (via the manual flushing mechanism) the tank begins to empty into the
bowl.
The flap is now surrounded by tank water and floats in an "open position" as
the tank empties.
The flap is "spring loaded" so than it can sink to its original "closed
position" when the tank water level has decreased sufficiently. Its lower
surface is again in air and the water pressure on its upper surface holds it
in place.