Up until firearms were invented, armies used archers for similar purposes. A well-timed volley, when one can't see the sky for the flying arrows, can have a demoralizing effect on the enemy in addition to the physical damage it is likely to cause.
In order for a volley to be well-timed, the archers had to take aim and shoot simultaneously, following a single order given by a single commander. Ever since the advent of firearms that order has been "Fire!"
Now imagine, say, a Roman centurion, his back to the archers standing in formation, waiting for his signal. He's watching the enemy's movements. Now he raises his hand. He waits. At last he judges that the right moment has come. He shouts a single word, sonorously enough to be heard by every single one of his archers ... What is that word?