This Speed- sensing device uses gears and flyweights to detect the changing load and adjusts the throttle accordingly. The mechanical governor works on balancing the centrifugal force of the flyweights and in return balances the equal and opposite radial force acting on these rotating balls which are termed as controlling force.
Suppose you are operating an engine at a light load applied to it. In this situation, the carburettor delivers small amount if air-fuel mixture to the combustion chamber. Now as the crankshaft spins, the centrifugal force created opens the flyweight. As they operate the pressure is applied on the governor cup and crank which are directly linked to the throttle valve. This action of the flyweights pulls the throttle to the closed position.
Now in other situation as the load on the crankshaft are increased leads to the slow spinning of the flyweights. This decreases the centrifugal force on the flyweights and leads to less pull of the throttle valve to the closed position and the supply of the working fluid increases.