Four rods are attached to each other with hinges at the vertices of a rhombus. Two of the sides are extended, with balls attached at their ends. The top vertex has a constant height. As you rotate the whole assembly, the balls move upward due to centrifugal force. The bottom vertex rises in proportion to the speed; therefore, it can be used to control the speed. A similar arrangement, called a flyball governor, was used by James Watt in his steam engine.