Flowing pressure on the unbalanced stem area puts an axial force on the pin. At set point, the pin buckles and the valve closes for a bubble tight seal. In case of a downstream line break, the mass velocity impinging on the lower piston surface will force the pin to buckle and the valve will seal closed. In case of fluid trying to enter transmission lines, the slug of fluid will buckle the pin when it hits the lower piston surface. ADVANTAGES: Only one moving part, has extreme accuracy and proven reliability.