When a horse is flexed at the poll, most of the bit pressure goes to the lower jaw. The tongue is pulled backward away from the upper teeth. At the same time, the bit pressure can push the cheeks into those upper jaw teething the area where these teeth are the sharpest and most damaging. The upper teeth cause more cheek problems, because the upper jaw is much wider than the lower. The result is sore cheeks, especially at the corners of the mouth.
Lesions indicate that the bit exerts too much pressure.
Stubbs described the pressure of the jointed bit as making the tongue into a triangular shape, with the potential of exerting severe force on the lower jawbones. When the horse's nose is pointed toward the ground and you pull back on both reins, much of that pressure goes to the lower jawbones. His approach to dentistry is first to make the horse comfortable in the mouth. If the horse fights the bit from the pain it causes, the teeth can be causing the pain. If the teeth are fixed, you've got a better horse.