Hi will try to explain. In old regulators direct acting ones, force of spring s1 and cylinder pressure are the only forces which keeps the valve allowing gas to flow from cylinder to regulator open. SpringS1 used to be adjustable in nature. In a situation where spring S1 was left untightened and flow meter is left open, the gas from cylinder would flow through the regulator in an uncontrolled manner, with no control over the extent of reduction in pressure. In such situation, spring S2 would keep the valve shut preventing gas from flowing from cylinder to regulator. Modern pressure regulators are indirect acting, that is force of spring S1 will oppose entry of gas from cylinder to regulator. And the force is not variable but is fixed. So the newer indirect acting ones do not need a spring S2 at all and it is only of historical significance.