Theres a fluid filled cylinder inside the clutch.
The "clutch" is filled with a viscous fluid. When the engine is cold, the fluid is thick and serves to couple the fan to the driving pulley, thereby driving the fan. As things come to temperature, the fluid warms up. When the fluid warms up, it gets thinner. The coupling effect between the fan and the hub is reduced, which causes the fan to rotate slower. A slower-speed on the fan improves fuel mileage. It does rotate slower when the engine is warm, but it never totally stops.