The horse senses even the lightest touch on the reins by the rider, but the bit is only one part of the message. A rider communicates with her horse via myriad "aids". The superior rider communicates largely through body language...the subtle shift of her weight in the saddle...a leg that slides back and squeezes...or verbal cues like murmured encouragement or soothing reassurances. It is the unskilled riders who primarily use the bit, usually because it is the only venue where the rider has greater strength than the horse: her arm strength exerted on a metal bar in his soft mouth. Unsurprisingly the bit quickly becomes an item of contention for the insensitive rider and her mount.
Despite what most people believe, the rider is not "making" a 1200 lb. horse do anything; rather, the rider is asking and the horse is saying "yes" or "no." Some riders, convinced as they are of their skill, constantly switch mounts, sure that if they find The One they will become the rider they imagine themselves to be. Some people also trade human mounts, convinced that The One will bring them happiness and everlasting love. Aside: Feel free to guffaw, chortle, snort or vomit now.
Little girls love riding horses, and it serves them well as preparation for adulthood. As the smaller of the sexes, a girl is unlikely to be able to muscle her way through life any more than she can push around a horse. Her safety will be maintained only through luck, good choices and effective communication, because Goddess knows she cannot rely on the benevolence of an animal who outweighs her and has little conscience. Because of the bit, the little girl will learn a little about power and how to wield it effectively. Equitation has much to reveal about communication.
To communicate via the bit, a rider must achieve a delicate balance called "contact." Contact is when the rider's intentions are effectively communicated via the reins to the horse as translated through the bit. Horses will avoid contact when the rider's communique's are confusing, inconsistent, irritating or painful. The horse is sensitive to the bit, which is essentially a tuning fork held between his teeth. Careless reining by a rider will cause discomfort to the horse, which he will seek to avoid. There are two ways that a horse will avoid the pain of clumsy or abusive reining that are oddly analogous to human behavior: going "above the bit" or "behind the bit."
When a horse goes above the bit, they brace upward against the bit and the pressure exerted by the rider on the reins, making heavy contact with the bit and numbing the rider's signals. Think of holding that tuning fork in a closed fist; you can strike it, but it won't resonate. The technique has limited success because a person can outmuscle the horse's mouth after some physical struggle. The pissing match that is "going above the bit" always ends badly for the horse. He will be hurt and dispirited.
After too many battles going over the bit, a horse will avoid contact by coming "behind the bit". The horse may appear to be "on the bit" by assuming a posture that looks compliant, but is actually evasive. By pretending to go along, the horse frees himself from the painful fumblings of his unskilled mount.
Usually the shitty rider who caused the problem moves on to inflict her miserable technique on other horses. Sometimes she takes the time to learn to ride. So what is next for the horse?
Horses--and women--that go over the bit often respond well to gentle handling. They hadn't given up, and maintained a willingness to engage. When a rider lightens up on the reins the horse comes onto the bit and reengages. It's a beautiful thing to witness. A horse, once high-strung and prone to outbursts and dramatic displays of frustration becomes sweetly engaged, willing and a delight to his rider. Many women respond this way as well.
A horse--or a woman--that goes behind the bit is a bit trickier to rehabilitate. A steady hand is required, yes, but the problem is systemic, and will require more from the rider than simply not being an asshole. A gentle hand alone will not inspire contact. The horse needs to re-engage, to be willing to try. In riding, this is done by creating forward impulsion with the rider's leg. I don't recommend trying this with a woman.
But back to the horse: the leg drives the horse forward into a hand that "catches" the forward movement, gently, providing a safe place to land. It's a delicate balance that will take some time to figure out: how much impulsion is too much? How heavy or light must the hand be? It can be frustrating for a rider to try, fail, try again, hit it, then fail the next time. But isn't that life? Doesn't it give the journey some excitement? Some texture and flavor?
Because one day when you least expect it, the horse will follow the contact forward, and down, if the reins are softened by the rider. If the horse follows, it is, so to speak, the horse that chooses to touch the rider with its mouth. And it's magic.
Some dressage for you: