fluid drive

Are you talking about this? http://en.wikipedia.org/wiki/Fluid_Drive

whats the principle behind fluid drive mechanism?

Fluid Drive, if you mean What Chrysler had in their 1948-50 Plymouth/Dodge/Chryslers. They had a clutch like a standard, but was only required to get the vehicle moving at a speed ussually for 1st gear, at this point a fluid coupler would have enough oil pressure to cause the automatic transmission system to shift to 2nd gear & 3rd gear. So The Clutch was only used for Changing Directions, as in Reverse &/or starting out from standstill. It was basicly a low pressure automatic, with clutch assist.