I ride in hilly country. Granted, not as steep as the ones in the old films and not at that speed. But the one thing I noticed with the films featuring the US cavalry was all the head tossing on the part of some horses. I ride with loose reins. But particularly going downhill. Based purely on my experience of riding in hilly country, a horse with a free head is where the balance begins. All that head tossing indicates that the horse wants to set his head free. Coincidentally, I also noticed in all the old cavalry films featured in this discussion, that all the riders probably fit the 20% of the horse's weight carrying limit. It is a fair question to consider whether that aided the horses. All horses regardless of whether the rider was sitting forward, backwards or neutral, made it downhill.