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
Applications
hydrogen