As a horse person, I just wanna point out that the horses in the relevant scene in Gandhi were not “charging”. A cavalry charge takes place at a full gallop (for you non-horse-persons, that means “running very very fast”–think Kentucky Derby). The horses in the movie were being urged forward at a walk, as all “crowd control” police horses are, because horses panic easily and they will bolt (means “run away out of control”), so you keep them under control, and moving slow, at a walk, when you try to break up a crowd with horses.
And yes, horses are very picky about their footing and yes, if a horse has a chance to think about it, he will refuse to walk forward into something very strange and unknown, such as a carpet of human bodies.
Some horses will even refuse to walk up something as seemingly innocuous as a wooden loading ramp.
However, a cavalry charge is something quite different from what was portrayed in the movie. In a cavalry charge, you get the horses up to full speed, with kickin’ and whoopin’ and hollerin’ and whips and spurs and things, and then they’re basically panicked, and yes, then they’ll stampede right over anything and everything, human bodies, wooden loading ramps, whatever. And it doesn’t require any special “battle” training, either–as any rider will tell you, all horses, being prey animals and genetically designed to outrun predators, are hard-wired to bolt at a moment’s notice, and all it takes is a certain amount of kickin’ and whoopin’ and hollerin’, plus the example of other horses around them also panicking and bolting, and you’ve got yourself either a stampede, or a cavalry charge, depending on what you do with it.
But that’s not what was happening in the movie. And no, none of the great historic cavalry charges would have come out any different if the charg-ees had lain down. The galloping horses would simply have trampled them.
Yes, the riders in the movie were yanking back on the reins, but I interpreted that as owing to the director’s need to show the horses in distress. Otherwise, the scene, which probably read in the script as something like, “Horses move up to front rank of bodies and refuse to go farther” would have simply looked like the horses walking up to the front rank of bodies and then stopping, like, “Oh, hey, I can’t walk any farther because there are human bodies all over the ground and I’m certainly not going to step on anything weird like that.” And then they would just stand there, heads drooping down, with sullen and bored looks on their faces.
You have to saw on the reins to get their heads up and jerking around, to get the effect of their eyeballs rolling in distress. Actually they weren’t in distress, just startled and baffled, if you know how to read horse body english. “Why is my rider doing this?” because the riders were also kicking them. If you saw on the reins like that without also kicking, it’s the signal for “back up” and we would have seen the horses backing away from the carpet of human bodies. So the combination of the riders kicking, which means “Go”, and sawing on the reins, which means both “stop” and “back up”, combined to make a splendid cinematic effect of equine distress.
P.S. It doesn’t hurt the horse to rock the bit in his mouth like that, it just makes him toss his head very dramatically. Which no doubt was what Attenborough wanted.