Erik the blades of the fan are flexible and bend forward in the water and hit the radiator.
When they heat up the goo inside stiffens and acts as a clutch.
I don't like them. They rob you of power at normal driving speed when they are not needed, and don't pump enough air when you are crawling at idle. Viscous fans are only nice if you tow regularly.
An electric fan engages only when needed, with the added bonus that you can switch it off with an overide switch when wading. And if you are concerned about having enough air you can add another smaller "pusher" fan on the front of the radiator.