Originally Posted by OffroadFrance

Put simply, yes it works solely by temperature based upon fluid dynamics. As the oil inside heats up in theory it becomes more viscous and 'grips' better the two parts of the void. As it cools it becomes less viscous and the fan slips more and therefore rotates less. All very fickle in practice.

This is a Jag viscous fan and explanation which is very similar if not the same.

Jaguar V12 Fan Clutch

My only issue with electric fans are that if you are in the middle of nowhere and the fan expires you can easily 'lunch' your engine and can do nothing about a burned out electric motor whereas a viscous fan can be modified to run 'locked up solid' and maintain radiator/engine cooling. The only other concept is twin lead lag electric fans where each runs at a duty greater than 50%, ideally 60-65% of total duty and if one expires the other gets you out of trouble at least even if you have to stop occasionally to cool the system down. More ideally would be that they are both 100% of duty running lead lag with an auto speed modulator possible solid state electronics coupled to the engine temperature, complicated, yep very, until you get seriously into hi-end thermocouples and electronic controllers.