text: how does a fluid drive coupling works?   propellers, or fans, or whatever you want to call them.   a propeller that is attached to the drive mechanism spins a fluid (liquid or gas), which in turn spins a propeller that is attached to the load. the pitch of the blades of the two propellers can produce the same effect as gear reduction, so the two propellers can spin at different speeds.  when fluid drive is used on an automobile, the drive propeller (attached to the engine) spins faster than the load propeller (attached to the wheels). thus the engine can run at the high speeds it needs to produce maximum power output without sending the car off a cliff.