They were simply a hydraulic gear pump inside a drum/reservoir, in series with the drive mechanism. NO motor, just a pump and a restictor mechanism.
To go FAST, you moved a lever that operated a valve that closed off the output of the pump, effectively locking it up, so the pump body/drum turned along with the input shaft (less a tiny amount of slip).
To slow down, the valve was opened to a degree that allowed enough oil flow out of the pump discharge back to the reservoir to let the pump "slip" a certain amount, to get the desired output speed.
There was (of course) no feedback mechanism, and they "slipped" more at a given lever setting as the oil warmed up, requiring the operator to fiddle with the control to maintain exact speed.
The drums were deeply finned to aid in cooling the oil as much as possible.
Bottom line is the were VERY crude and inefficient, and inaccurate in speed control, compared to a modern hydrostatic unit.
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