for any automatic transmission car, including coure i try to keep it as simple as possible:
as you know auto shifts transmisson according to load on engine and accelerator pedal, you have options of 1 (1st gear only), 2 (1st and 2nd gear) and D (drive 1 , 2 and 3) plus O/D 1, 2, 3, 4 (over/drive) the gear ratios are designed in a way that D of auto is somewhere in between 3 and 4 of (5 speed gearbox). and O/D is close to 5th gear of (5 speed gearbox. triptronic and 4 speed auto from honda and other companies is similar giving more options to driver.
But the important point is automatic gearbox does not downshift a gear on its own,, unless you want to accelearate car using acceleartor padel. so going uphill or downhill, driver needs to use "2" instead of "D" to keep car in engine control and avoid brake overheat and skid..... if you plan to drive slow around 40 km/hr approx on steep climb or downhill. best is to keep car in higher gear instead of putting it in D that will loose the engine power away from car often, not recommended at all.
same applies to sand/mud/hard rain/snow/ice/ pulling another vechicle/load or other similar road conditions.. where driving at slower speed required.,,,,,,,,,drive safe!