Please take some pics and post them, so we can see how it works. Even if you find the answer immediately when you look inside, it will make a useful archive. However if it is similar to the Rover auto drive, it is just a system of belts and counter-rotating pulleys, and all the pedal does is engage one or other of two clutches, attached to the pulleys. "Auto" drive means the clutch is slipping unless you are pressing the pedal firmly and running the engine slowly. I expect the outcome to be a worn out forward clutch in a fairly short time.
If it is a variable ratio drive in there instead of a slipping clutch, the words auto drive are being used very differently by different mower brands.
