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Reference reading:
http://www.brickboard.com/FAQ/700-900/Cooling.htm#Thermostatic_Fluid_Clutch_Test
It's sometimes referred to as a viscose coupling. At low RPM's the fan turns at something close to the water pump speed. As the RPM's increase, the aerodynamic drag on the fan blades also increases and the fan clutch will "slip" somewhat. The greater the RPMs (and fan drag), the greater the difference in water pump RPM vs fan RPM. Eventually the fan hits a terminal speed regardless of how fast the engine turns.
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