Refer to figure.
An AC generator driven by a constant speed unit is also known as a Constant Speed Generator Drive System, CSDU, or CSD. The main function of this system is to maintain a constant frequency, sometimes referred to as speed. In other words, it is a hydromechanical unit that turns the physical motion of oil/fluid into an AC or DC current. The output of the hydraulic motor, and therefore the speed of the motor is dictated by the angle of the swashplate. The angle of the swash plate will either limit or increase the oil flow towards the hydraulic and in turn affect the rpm of the engine. The angle of the swash plate is controlled by the speed governor, which in turn is controlled by the load controller. The load controller senses the output frequency of the alternator and will adjust the swash plate accordingly to increase or decrease the torque output to the generator.

The CSD can be disconnected from the engine drive shaft at any time during the flight. However, only the engineers can reconnect the CSD, on the ground, with the engines off.