To answer these questions, it is helpful to have a general understanding of how the muscles and bones work. To be able to see which direction the bones are moving in and to know wether or not the movement is correct. With this knowlege you will be able to bring out the best in your horse and lessen your chances of injury.
The jaw moves left and right and the lower jaw moves down and up. In order for the horse to properly chew its food, the teeth need to be in alignment. Because the teeth are contiunually growing, they can grow out of balance. The teeth on the left can grow longer than the right (this usually means the horse chews on the right side of their mouth), or the teeth in the back of the mouth can grow longer than the front (which is called a wedge). Then the incisors (front teeth), as they grow, develop an over bite. Without the aid of a repituable equine dentist the teeth become a significant issue to the horse for both good body mechanics and good health. If you are figting with your horse and have never had their teeth done, then stop and at least have their mouth looked at. As you read on about the teeth and about the body, I hope it will make more sense as to how good dentistry gives the horse the ability to maintain their natural head movement.