A curb chain, or curb strap, is a piece of horse tack required for proper use on any type of curb bit. The main use of the curb chain is to enhance and control the lever action of a curb bit.
Additionally, it helps to keep the bit steady and in place within the mouth. The tightness of the curb chain has a great effect on the action of the bit.
If the bit is used without a curb chain, it loses its leverage action. If used with a loose curb chain, it allows the shanks to rotate more before the curb chain is tight enough to act as a fulcrum and exert pressure.