The coupling can start empty, with all the oil expelled from the impeller, so the motor starts unladen. After the motor begins running at full speed, the impeller is filled with oil to start the driven side and begin accelerating it. Torsional vibration generated by the engine is absorbed, so uniform torque is transmitted, while load overtorque is handled by slipping.
Charging and draining the oil inside the impeller makes it act as a clutch, so even intense repetitions of on-off running will never cause damage, in contrast to a mechanical clutch. This clutch action can be used to easily achieve trial running of the motor in isolation. In multiple drive systems, motors can be stopped as required, and only the motors that are required can be left running at maximum efficiency.