So the question came to me while reading this TIL, in regards to the largest cavalry in history. 18,000 men strong, it was compared to the Rohirrim Charge in Return of The King, with only a third of the strength.
Using that video as an example....how the hell did anyone beyond the front line know what to? Théoden's giving his orders to his generals, who disperse, but there's no godly way they could relay that to their entire companies in the 2 minutes he's giving his rousing speech-and no one would be able to hear that. The horns were merely the charge order, not the specific direction.
Now granted, that's a movie. But it was reportedly based on other historic cavalry charges, and to circle back to the original point of the question-the largest one in history was three times the size of the Rohan flood. So beyond listening for a horn and charging full tilt into a wall of spears, how were any tactical decisions relayed?
Also, IE is auto-correcting Théoden's e....interesting....
Preface: this applies mostly to early modern cavalry, but is not necessarily wrong for cavalries of other eras.
Well, first things first, cavalry forces that conducted large scale charges drilled extensively. This meant that they practiced charges and maneuvers in smaller formations often. This allowed the officers to gain experience commanding their group of soldiers.
Essentially, a charge of that size would have to be planned ahead of time, and officers would be given orders. They would know their general area of responsibility, and they would be responsible for moving their men to that location.
In video game terms, think of grouping all of your individual cavalry units into hotkey groups. You have 100 horses per group, and you have 4 groups. Instead of telling all 400 where to go individually, you tell group 1, 2, 3, and 4 where to go, and then let the groups get there independently.
If the leader of all the groups needed to communicate something to everybody, he would have a few ways to do so. The most efficient way would be to use a bugle call. Bugle calls weren't just one sound- there were tons of different calls, which signaled different messages. The officers and men would hear the bugle call and act accordingly. In a cavalry force that large, they might have to rely the bugle call with another trumpeter.
Another way to communicate would be to use flags. Flags have severe limitations within the cavalry and were somewhat prone to miscommunication, but they were not without place.
The cavalry officers might also communicate with hand signals. One officer sees the signal, then relays it to the next few officers. This could also be done with movements of the sword held above the head.
Alternatively, the general could send relay riders to the different groups. The relay rider would basically shout orders, and the army would play a large scale game of "telephone."
As you can see, communication here was not exactly precise- often, they used rough signals to relay basic messages quickly. The general had to have a talented group of officers to help rally his army and complete his tactical goals. Good officers made the difference between a successful attack and a failed one.
You could think of it kind of like American Football- the players know all of the plays, and they've practiced the plays repeatedly. However, they will not communicate with the coaches directly during the game. The quarterback might see a signal from the coach on the sidelines, and then the quarterback will tell the players what play they're using, and then he will help make sure the play is executed correctly. If something goes wrong, he can improvise and fix the play, or exploit a weakness in the other team and change the play on the fly. That's why good quarterbacks get paid a lot of money.
And finally, if the individual cavalryman didn't know what to do... he'd keep his head down and follow the guy in front of him and trust that his officer wasn't a moron.
I hope that helps!