A snaffle is not necessarily a bit with a jointed bit mouthpiece, as is often thought. A bit is a snaffle because it creates direct pressure without leverage on the mouth. It is a bit without a shank. Therefore, a single- or double-jointed mouthpiece, though the most common designs for snaffle bits, does not make a bit a snaffle. Even a mullen mouth (a solid, slightly curved bar) or a bar bit is a snaffle.
The snaffle bit works on several parts of the horse's mouth; the mouthpiece of the bit acts on the tongue and bars, the lips of the horse also feel pressure from both the mouthpiece and the rings. The rings also serve to act on the side of the mouth, and, depending on design, the sides of the jawbone.
A snaffle is sometimes mistakenly thought of as "any mild bit." While direct pressure without leverage is milder than pressure with leverage, nonetheless, certain types of snaffle bits can be extremely harsh when manufactured with wire, twisted metal or other "sharp" elements. A thin or rough-surfaced snaffle, used harshly, can damage a horse's mouth.
Curb chains or straps have no effect on a true snaffle because there is no leverage to act upon. English riders do not add any type of curb strap or curb chain to a snaffle bit. While some riders in western disciplines do add a curb strap to the rings, it is merely a "hobble" for the rings, has no leverage effect and is there only as a safety feature to prevent the rings from being pulled through the mouth of the horse, should the animal gape open its mouth in an attempt to avoid the bit, an outcome prevented in an English bridle by the presence of a cavesson noseband.
The snaffle differs from the pelham bit, the curb bit, and the kimberwicke in that it is a non-leverage bit, and so does not amplify the pressure applied by the reins. With a snaffle, one ounce of pressure applied by the reins to a snaffle mouthpiece will apply one ounce of pressure on the mouth. With a curb, one ounce of pressure on the reins will apply more - sometimes far more - than one ounce of pressure on the horse's mouth.
There are many riders (and a remarkable number of tack shops) who do not know the true definition of a snaffle: a bit that is non-leverage. This often results in a rider purchasing a jointed mouthpiece bit with shanks, because it is labeled a "snaffle," and believing that it is soft and kind because of the connotation the snaffle name has with being mild. In truth, the rider actually bought a curb bit with a jointed mouthpiece, which actually is a fairly severe bit due to the combination of a nutcracker effect on the jaw and leverage from the shanks.
A true snaffle does not have a shank like a pelham or curb bit. Although the kimberwicke appears to have a bit ring like a snaffle, the bit mouthpiece is not centered on the ring, the ring has slots to place the reins in a specific location, and it is used with a curb chain, thus the ring acts like a bit shank and creates leverage, making it a type of curb bit.
A snaffle also will not be able to slide up and down the rings of the bit or cheekpieces of the bridle, as this would place it in the gag bit category.
The mouthpiece is the more important part of a snaffle, as it controls the severity of the bit. Thinner mouthpieces are more severe, as are those that are rougher.
There are several types of rings that affect the action of the bit.
The horse should keep its mouth closed over a properly-fitted bit (slight chewing is acceptable and a sign of relaxation) and hold its head quietly. A bit may need to be adjusted either higher or lower until the horse shows no signs of discomfort. The height of the bit in the horse's mouth has little significant impact on its severity. Some riders mistakenly think that raising or lowering the bit increases its effect, but this is not correct. The bit is at its most effective when properly adjusted. Improper adjustment only causes discomfort, not increased control.
Factors that affect the fit of the bit include both the length of the mouth overall, the length of the interdental space between the incisors and the molars where the bit rests on the bars (gums) of the horse's mouth, the thickness of the horse's tongue and the height of the mouth from tongue to palate. There is less room for error with a horse who has a short mouth, thick tongue and a low palate than with a horse who has a longer mouth, thinner tongue and a higher palate.
One of the important criteria when fitting the snaffle is that it does not hit the horse's teeth. The greater concern is that the bit not be so high as to constantly rub on the molars, which can cause considerable discomfort to the horse. A bit adjusted too low usually will not come anywhere near the incisors, even on a short-mouthed horse, until the entire bridle is at risk of falling off.
If the bit is adjusted too low (not touching the corner of the mouth), it is primarily a safety concern, though the action of the bit can also be altered and lead to discomfort. A horse can get its tongue over a too-low bit and thus evade its pressure, plus the action of the bit is altered and it will not act on the mouth as it was designed. Horses with a bit too low will often open their mouths to evade pressure and may chew on it excessively. In extreme cases, the bridle could even fall off if the rider pulls hard on the reins, hence raising the bit and loosening the cheekpieces, at the same time the horse rubs, tosses or shakes its head vigorously.
Many horses will "carry" a too-low bit themselves, using their tongue to hold it in the proper place. Some trainers, especially in western riding disciplines, consider this desirable and adjust a bridle a bit low to encourage this behavior. Other trainers, especially in English riding disciplines, prefer to hang the bit a little higher so it is in the correct position without need for the horse to move it there.
If the bit is too high (depending on the horse, at three or more wrinkles in the lips), it will irritate the lips, leading to callousing and a loss of sensitivity over time. However, the more immediate consequence is that the horse feels constant bit pressure and cannot get any release, even if the rider loosens the reins. This leads to the horse becoming tense in the jaw and resisting the bit. Most of all, if a too-high bit rubs on the molars, this discomfort will cause the horse to toss its head and otherwise express its displeasure at the situation, leading to a poor performance.
If the horse tosses its head or attempts to evade contact with a bit, improper fit is usually the cause, but other factors should be considered. A rider needs to verify with a veterinarian that the horse does not have a dental problem. Then bit fit and the type of bit needs to be considered. But finally, the skills of the rider may be a factor. Even the gentlest bit properly adjusted may still cause discomfort to a horse in the hands of a poor rider.
However, mouthpieces that are extremely thin, such as wire mouthpieces or those that are only 1/8-1/4" in thickness, are never mild. These can be damaging to a horse's mouth.