The Fluid Drive is a fluid coupler invented in the 1930s. It works just like fans, imagine having two electric fans placed on a table facing each other. Turn on the first fan, soon the second fan will start to turn as the breeze is hitting it. This is the fluid coupling principle, there is a sealed housing, with fan blades on its inside and then the driven member which rotates freely. This part is attached to the engine while the second part to the transmission. When you start the engine at first there is a lot of delays but as the car moves faster they move together. Normal cruising, there is a slippage of 2 or 3%. It was usually attached to a 3 speed or 4 speed and was a great innovation for it solved the problem of working the clutch.
The other innovation was the Torque converter which was similar to the fluid coupling though it had an additional third fan in between the first two. Its blades were organized deflecting the oil causing it to hit the driven member with higher force. And the effect of this was increasing power output. The Torque converter increased torque 2.6 times at stall, similarly to gearing down transmission 2.6:1. The initial torque converter transmission was Buick, Dynaflow, and Chev Powerglide. They were 2-speed transmissions built to all work in high gear for emergencies. They start in high and never shift.
Chrysler decided to make the torque converter optional in place of the torque converter unit. Combining the Hemi V-8 with torque converter plus four-speed would lead to a stunning performance for 1953. The Fluid drive is a sealed and self-contained unit, to check its oil level or top up you use a hole in the floorboard. The torque converter needs a different oil supply, to check or feed oil there were oil passages across the engine block and bell housing. They require more oil for an oil change. Normally gearshift quadrant has a Fluid Drive or Fluid Torque Drive markings.