It (the cline) allows muffler (4S) or crankase(2S) pressure to build in the tank thru a one way valve in the pressure line and a fuel regulator which only allows fuel to flow to the carb as the engine demands. I have one in a zero with a tank 2 1/2 inches too high with an inverted 4S 90 engine and it works great. No fuel gets thru when filling and therefore no syphoning.