Fan clutch is "clutched" at start up because the oil inside settles due to gravity and falls into both chambers.

When operating, centrifugal force keeps the oil in one chamber until the thermostat opens, then it's able to flow into the other chamber to transfer power to the fan.

If it does not "declutch" then the thermostat is stuck or the little valve the thermostat works has been screwed up somehow. The thermostat is on the side facing the radiator, and it's usually an open coil that is free to collect lots of dirt and crud.

Check this out: http://www.nichols.nu/tip482.htm

97 T100 4x4
97 328i
95 540i restoration project