Gag Bits

The gag bit works on the horse's lips and poll simultaneously. The pressure on the lips tends to make the horse raise its head, which is useful for a horse that tends to overextend. Polo players, who necessarily ride with the reins in one hand and cannot make instant fine adjustments, often use a gag bit with draw reins. The reins are adjusted such that, when the horse tucks in its nose, the gag action ceases automatically.

Gag bits are used mainly for horses that are strong pullers or for horses that need retraining. Gag bits are most commonly seen in polo, eventing (especially for cross-country), show jumping, and hacking, mainly for increased control at times where a horse may be excited or try to run off with the rider.

They are not permitted at any level of dressage, since dressage riders are trying to get the horse come down onto the bit, and want to encourage the horse to accept contact. Additionally, the horse is supposed to be completely submissive in dressage, and a gag bit gives the impression that it is not. Gag bits  are also never seen in the hunter arena, again because they wish to portray that the horse is an easy ride, and because they want a long, relaxed frame with the neck stretched out, rather than a high neck.