As the RAVE manual says, when you first start the engine the fan will move and can clearly be heard as the fluid will all be gathered in the bottom of the unit causing drag. Centrifugal force then moves it into the outer chamber after a few seconds and there is less drag and the fan speed reduces. As the engine temp increases the bi-metallic spring moves a lever opening a valve to allow the fluid to cause drag between the two plates and thus increases the fan speed.