viscous fan not turning

I recently posted a prob "hot (ish) temp gauge, well today I was having a peep at the fan and it only went around when the engine revs were increased but not at all if I held it with my finger (even at high revs)
Is this arrangement normal or have I stumbled on a possible cause of my original problem?

First don't try that trick again.
The fan may suddenly turn and mash your fingers.

The viscous coupling is a FREE-WHEELING AUTOMATIC HEAT SENSITIVE CLUTCH.

It is designed NOT to drive the fan until the alloy finned hub on the front gets HOT. This saves power (4 to 5 bhp sometimes) and fuel, and makes the engine warm up more quickly. In cold weather the fan may never actually be driven for months, though usually it will turn a bit.

When the engine heats up the thermostat opens and lets very hot water reach the radiator. The warm / hot air flow from driving (or if you are parked, from the fan slowly idling) will come back from the radiator and warm the finned hub of the fan coupling. This allows a silicon goo inside the finned hub to flow into the clutch pieces, and lock them up, which means the fan belt starts to drive the fan positively.

It works when it works, but can be slow to react, and you have no control over it. An electric fan is best, with an over-ride switch on the dash so YOU can turn it on whenever YOU want.

CharlesY