On the bit

The phrases "on the bit", "behind the bit" and "above the bit" are equestrian terms used to describe a horse's posture relative to the reins and the bridle bit. A position on the bit is submissive to the rider's rein aids, given through the bit. When a horse is behind the bit, the head is tucked too far down and rearward. If above the bit, then the head is too high.

Being on the bit requires the horse to engage the hips and raise the back, which it cannot do when its head is pulled rearward. The neck is connected to the shoulders, and impeding the shoulders prevents extension of the forehand. This will cause the horse to hollow its back. A horse is properly placed, on the bit, by creating impulsion (pushing power) from the rider's driving aids, and then containing this forward energy in the hands, via the reins and bit. Impulsion causes the horse to engage its hind end, lift its back, and finally (when it becomes submissive and accepts contact with the bit, without resistance) results in the horse flexing at the poll, maintaining an elastic contact that is equal on both sides of the bit. The horse stretches over its topline and follows the bit's contact forward and down. Being "on the bit" is more than just a fancy head position; seesawing on the bit causes tension throughout the body. On the bit is synonymous with "on the aids", where the horse is relaxed, using its back and hindquarters, and is responsive to the aids without tension.

As a test, the rider can soften contact, and the horse will maintain the pressure and follow the bit downward. The horse does not have to have its head perfectly perpendicular to the ground; it is acceptable, in dressage tests, to have the nose slightly in front of the vertical.

A horse is not "on the bit" only because its head is held "at the vertical," or perpendicular to the ground, as a horse can maintain this headset while remaining stiff, heavy on the bit, and unresponsive to the rider's aids. The vertical headset is not a guarantee by any means that the animal is truly on the bit, and many novice riders achieve the vertical headset, while losing the impulsion from the horse, because they ride "front to back," or pull the horse's head down in an effort to make the horse appear to be accepting the aids. This is also sometimes seen when the horse is ridden in certain gadgets, such as draw reins, especially if the rider is not skilled enough to correctly use the piece of equipment.

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Wikipedia