On the Cavalry Commander (9.9-9.10)

As for bits, the smooth are more suitable than the rough; but if a rough one is used, it should be made to resemble a smooth one by lightness of hand. It is also well to accustom oneself to sit still, especially on a spirited horse, and to touch him as little as possible with anything other than the parts that give us a safe seat by contact.

It should also be known that a horse can be taught to be calm by a chirp with the lips and to be roused by a cluck with the tongue. And if from the first you use with the cluck aids to calm him, and with the chirp aids to rouse him, the horse will learn to rouse himself at the chirp and to calm down at the cluck.