Not a dumb question at all mate but the answer is that the viscous fan has a heat sensitive spring in the centre of it, when the spring heats to a set temp it expands and compresses the fan hub and it engages.
The fan will spin all the time but without load, when it engages you can hear it load up and spin harder until the engine cools below the trigger point then it lets to as the fan spins without load again.

