The mechanical governor in an engine uses flyweights and gears in the crankcase to sense the speed to detect changes in the load and increases or decreases the throttle accordingly. According to Briggs and Stratton, a governor is like a car's cruise control system, as it keeps the engine running at the same speed.
A governor in general is like a tug of war between two springs called governor springs. One of the springs tries to pull the throttle open, while the spinning crankshaft tries to close the throttle during an increase in load. An example is when a lawn mower is taken from pavement to grass. The grass, or increase in load, causes the crankshaft to slow, but the governor spring is still trying to pull the throttle open, so power is increased. A mechanical governor is made up of four parts, which are the governor crank, gears, flyweights and linkages. If someone is using a small engine under light load, then not much gas is needed. As the crankshaft reaches higher speeds, centrifugal force opens the flyweights, which apply pressure to the governor cup and governor crank pulling the throttle closed. When the load increases and the crankshaft starts to slow down, the flyweights close, which releases the pressure and increases the throttle to compensate.