The curb bit acts with leverage. As you adjust the reins to feel contact with the horse's mouth, the bit shanks move toward the horse's chest in a 45-degree angle. The lever presses on the tongue and bars, as the curb chain acts as a fulcrum on the jawbones. The length of cheek above and below the mouthpiece determines the curb's severity. A curb's ratio, or the comparison between the two lengths, calculates its action.
Most curbs have a port, a slight arch in the middle of the mouthpiece.
A straight mouthpiece can be more severe, especially with a long shank. In either snaffle or curb, a thinner mouthpiece tends to increase severity. A thicker diameter can spread pressure over a larger area. Styles like the Pelham and Kimberwick combine the curb and snaffle actions. Many trainers claim that these hybrids don't succeed in a clear communication, but some horses go best in one of these models.