The bi-metallic strip is what bends according to temperature and locks the fan or allows it to freewheel. Also keep in mind that it takes high temp along with centrifical force in order to lock the fan. You will still be able to turn the fan when the engine is off with your hand even when it is over the temp at which it should lock up.
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record