Simplified Operation of the Governor:

The governor controls the fuel rack position by a mutual action of the hydraulic piston and a set of mechanical flyweights that are driven through the engine blower shaft. Below Figure gives a description of a functional diagram of a mechanical-hydraulic governor. A position of the flyweights is determined by the speed of the engine. As the engine speeds up or down then the weights move in or out. The movement of the flyweights, because of a change in engine speeds and moves a small piston (pilot valve) in the governor's hydraulic system. That motion adjusts flow of hydraulic fluid to a large hydraulic piston (servo-motor piston). The huge hydraulic piston is connected to the fuel rack and its motion resets the fuel rack for increased/decreased fuel.

Figure: Simplified Mechanical-Hydraulic Governor