HST tractors will normally creep at a slow speed when on hills. This is because an HST is a fluid drive and inherently there is some internal leakage that allows the HST motor to turn when back-driven by the wheels. With the engine running, which allows the leaked oil to be replenished, you can creep down hill all day without harming the tractor.
Now if the tractor isn't running, the internal leakage is not being replenished, and after a short while (many seconds to a few minutes) of slow creeping there will no longer be any oil to resist the HST motor and the tractor will take off down hill at a higher rate of speed. This can damage the HST after a long downhill run. If you want to try it on a short hill for your education, a few feet travel isn't going to hurt it. Just be prepared to use the brakes to stop.
Towing an HST tractor with the gear selector NOT in neutral also will cause the oil the oil to leak out internally eventually resulting in HST damage. Putting the gear in neutral disconnects the HST from the axles and allows you to tow to whatever speed and distance is recommended in the operator's manual.