The empty space between a horse’s incisors and molars. The soft gum tissue in this area is sensitive. The bit is placed so that it rests in this part of the mouth. Pressure exerted by the bit pressing on the bars causes discomfort and is used to slow a horse’s pace, change his direction, or change his head or body carriage.
Young horses still have wolf teeth (tiny tusks visible in the photo above), which can cause discomfort when the horse is wearing a bit. (see Horse Teeth)
Please comment to add to this description.