The mouthpiece of a snaffle can be jointed or solid. A snaffle is customarily used with a brow band headstall that has a throatlatch. Often a noseband is used with a snaffle. The snaffle is useful for teaching a horse to bend his neck and throatlatch laterally so that he can be turned in both directions. It is also useful for teaching a horse to flex vertically in the lower jaw, at the poll, and at the neck muscles just in front of the withers.
Vertical flexion is necessary for gait and speed control as well as for stopping.
The bars are the flesh-covered portions of the lower jawbone between the incisors and the molars. This is where the bit lies. It is the action of the snaffle bit on the bars of the horse's mouth that produces vertical flexion. With a regularly configured snaffle, when one rein is pulled out to the side, let's say the right, the bit will slide slightly through the mouth to the right and the primary pressure will be exerted by the ring on the left side of the horse's face. This will cause him to bend laterally and turn right.