The main use of the curb chain is to control the leaver action of a curb bit. Additionally, it also helps to keep the bit steady and in place within the mouth, and helps to hold a lip strap in place.The curb chain applies pressure to the curb groove under a horse's chin when the curb rein of the bit is used. When the curb rein is pulled, the shank of the bit rotates back towards the chest of the horse and the cheek of the bit rotates forward .
The curb chain is attached to the hooks at the end of the cheek, so, as the cheek moves forward, the chain is pulled and tightened in the curb groove. Once it comes in contact with the curb groove of the horse it causes the cannons of the bit mouthpiece to push down onto the horse's bars, thus amplifying the bit's pressure on the bars of the horse's mouth.