Hydrostatic transmission (HST) usually accepts rotary power from a prime mover ( like an internal combustion engine) and transmits that energy to a load. All of this has specific operating characteristics.
The hydrostatic transmission, in addition, has to regulate torque, speed, power and, in some cases, the direction of rotation in the process.
You can see a HST whenever there is a hydraulic pump connected to one or more hydraulic motors. This transmission is preferred over shifted gear transmission usually because of its stepless way in which the speed ration of the HTS is changed. Check it out!