Hydrostatic transmissions offer many important operating features which other forms of power transmission do not. Two types of systems are used: open circuit and closed circuit. In an open-circuit system, the flow path of the circulating fluid is not continuous, being interrupted by the reservoir. In a closed-circuit system, the flow path is uninterrupted; the fluid flows in a continuous, uninterrupted path form the pump outlet port to the hydraulic motor inlet port and then directly back to the inlet of the pump. Some of the larger systems use a servo-controlled actuation mechanism to vary the position of the pump stroke control mechanism. To provide a more detailed and clearer understanding of how closed-circuit hydrostatic transmissions function, the following step-by-step description is presented. A variety of controls is available for providing the necessary displacement control needed for hydrostatic transmission operation, ranging from direct operated to servo controllers.