 On two-stroke engines, the operation of the power valve is the reverse of normal — it is normally "on" and at a set rpm it is turned "off".It is activated at high rpm to extend the engine's rev range, capitalizing on a two-stroke's tendency to rev higher momentarily when the mixture is lean.Alternately to employing a power valve, the carburetor may utilize a "metering rod" or "step-up rod" system to enrich the fuel mixture under high-demand conditions.