Rollers are a popular addition to many styles of mouthpieces today. Sometimes as little as a rotating disk set inside the center oval of a double jointed snaffle, or as much as a series of rollers along the joints or arms of the mouthpiece, the general idea of rollers is to encourage the horse to play with the bit. By moving their tongue under the bit, horses can become more relaxed in the tongue and jaw, leading to better acceptance of the bit. Whether or not rollers actually encourage such movement is, of course, debatable. However, some horses may be stimulated by the rolling action, so it may be worth trying with horses that tend to set their tongues and don’t salivate much.
Some designs can be problematic in that the roller action may lead to pinching; this can be tested by placing the bit over bare skin and applying pressure to the rings to simulate the action in the horse’s mouth. If you experience discomfort, it is likely that your horse will too. It is also important to check on current rules specific to rollers in your discipline, should you wish to compete using such a bit. Many types of rollers are prohibited in USEF competitions at this time.