Just as food in the mouth stimulates digestive system reflexes, so also does a bit. A bit signals a horse to "think eat". Yet a horse at exercise needs to "think exercise". The bit stimulates digestive system responses, whereas exercise requires respiratory, cardio-vascular and musculo-skeletal system responses. Eating and exercising are two incompatible and mutually exclusive activities. Horses have not evolved, anymore than we have, to eat and exercise simultaneously.

For the purpose of eating, the horse responds with an open mouth, reflex salivation, and movement of the lips, tongue and jaw. All these responses are stimulated by the presence of a bit. When a bit is in place, for example, retraction of the tip of the tongue tends to occur. If you place a pencil in your own mouth, you instinctively explore it constantly with the tip of your tongue. The horse does the same with a bit and, consequently, the root of the tongue often bulges upward and moves the soft palate in the same direction. The throat, which is capable of serving either swallowing or deep respiration (one or the other but not both at the same time) is, by the presence a bit, programmed for swallowing. Accordingly the tongue tends to be mobile and so also is the soft palate which lies on the root of the tongue. From time to time during exercise, the soft palate will tend to be pushed up by the root of the tongue or by reflex 'gagging'. This results in an enlargement of the throat's digestive portion at the expense of its respiratory portion. In other words, it enlarges the food channel at the expense of the air channel. The air channel becomes partially or completely obstructed, depending on the degree to which the soft palate is elevated. For swallowing, this elevation or, as it is commonly called, dorsal displacement of the soft palate (DDSP) is perfectly normal and acceptable.

For the purpose of exercise, the horse needs a closed mouth (the horse is an obligate nose-breather), a dry mouth (contrary to traditional thinking), and little or no tongue movement. The throat should be programmed for deep breathing, not swallowing. Accordingly, the tongue should not be on the move and the tip of the tongue should not be retracted. The soft palate should be immobile and lowered, to enlarge the respiratory portion of the throat at the expense of its digestive portion, i.e. to enlarge the air channel at the expense of the food channel. DDSP is normal for swallowing but abnormal and, in fact, disastrous for deep breathing. Predictably, episodes of DDSP are especially apparent in the racehorse, particularly in those racehorses (the Standardbred and, increasingly, the Thoroughbred) that race with two bits in their mouth. But DDSP also occurs in non-racehorses.

Use of a bit sends conflicting messages to the horse's nervous system and the confusion is particularly evident in its effect on the horse's wind. Human athletes could not perform well with a bunch of keys in their mouth.