Hi FS, not quite correct actually.
The way a viscous coupling works is that there are chambers within the coupling filled with a liquid. As the airstream coming off the back of the radiator heats up it affects a bi-metallic strip on the front of the VC, which in turn heats the liquid and turns it almost solid, to lock-up the coupling. This then gives full torque to the fan.
Nearly, but not quite.
A thermal clutch fan operates using silicone fluid as a viscous coupling medium. When the clutch is cool and disengaged, most of the silicone fluid is stored in a reservoir allowing your fan clutch to slip relative to your water pump shaft thereby spinning at a lower RPM than the water pump. This saves you money because the horsepower from your engine is not wasted driving a clutch fan when it's not needed. As your engine heats up, the thermal spring on the front of the clutch expands, which opens a valve allowing the silicone fluid to flow into the spaces between the clutch plates, and drive your fan at an increased RPM approaching the speed of the pulley eventually.
The silicone fluid flows back the the reservoir and stays there if the spring cools down and closes the valve.