I've driven many hydrostatic mowers and lawn tractors but I've never taken one apart until today!

The basic job of this transmission (and the concept of Hydrostatic power) is converting constant velocity power through a hydraulic pump into a matching hydraulic motor with a variable speed.

But to see the process and the parts...
That's a whole new level of understanding.

Here is a link that was shown to me with an excellent animation of the fluid flow process since it's impossible to operate with the transmission disassembled    • Hydrostatic Transmission  .

Enjoy!