1 No 
pressure

Static conditions 
(no flow)
 
The o-ring is relaxed
(Position 1).

2 Normal  
pressure   

Dynamic 
conditions (flow)

The o-ring subjected to the line pressure is compressed into the seating area which reduces the water passage (position 2).

3 High 
pressure

As the pressure increases the o-ring is compressed further into the seating area and reduces the water passage further (Position 3).
As the pressure decreases the o-ring relaxes and reopens the water passage (return to position 2 and 1).