The Shank
A curb bit is a leverage bit, meaning that it multiplies the pressure applied by the rider. Unlike a snaffle bit, which applies direct rein pressure from the rider's hand to the horse's mouth, the curb can amplify rein pressure several times over, depending on the length of the curb's bit shank. Shank sizes vary from the Tom Thumb (2 inches long) to more than 5 inches. The longer the bit shank, the more powerful its potential effect on the horse. For this reason, overall shank or cheek length, from the top of the cheek ring to the bottom of the rein ring, usually cannot exceed 8½ inches for most horse show disciplines.
Read more about this topic: Curb Bit