It is based on principle of equilibrium of forces, keeping it initially in
the closed position, since the maing spring force the diaphragm
together with soft shutter against the metal seat.
When the increase in inlet pressure overcomes the set point the
diaphragm set moves upward opening the valve and releasing the
gas to atmosphere or outlet pressure. The valve keep opening
until achieves enough flow to stabilize the pressure in the line.
After normalization of pressure in the line the diaphragm set
moves downward due of spring force, closing the valve