The way I think of it is:
The HST acts mostly like a gear shifter. As the pedal is depressed, gear selection changes; to go faster. But the engine is still developing the same amount of power. In a car, the RPMs are changed and so are the gears.
Example: If climbing a steep embankment requires the equivalent of a low gear, then the tractor will stall if it is in too high a gear. A high gear can be selected if the pedal is mashed to the floor boards.
At least that's the basic idea. The pedal also regulates the amount of torque available. So naturally, there is a position where the pedal supplies 100% of the available torque. I'm not clear exactly where this position is.
If you can create a situation where high torque is required, like climbing or digging, then play with the HST pedal when you are in the High gear range (it's easiest to see the torque response).
If possible, try this out:
1) Put the FEL bucket firmly and slowly into the side of a large dirt pile (one where you won't be able to move the dirt a whole lot). This will set up a high torque requirement. Make sure the bucket is firmly stuck into the dirt!
2) Place the gear selector into High range and put the RPMs to about 1800. This will provide an amount of horsepower that is good for digging but won't cause a safety issue.
3) Now smoothly depress the HST pedal to its maximum position. This will supply all the torque available per the amount of horsepower made at 1800 RPM. It will also change the hydraulic gearing from low to high.
4) You should see the tractor begin to dig into the pile and then stop its forward progress. The tires will probably spin somewhat. The RPMs should drop dramatically; the engine may stall.
5) Now perform the same procedure with the manual gear selector in Low range. This time, you should notice the tires spin much more and the bucket should dig into the pile much better.