The Science of Curb ChainsThe curb chain or strap is mounted in the chin groove under the horse's chin between the bit shanks of the bit. It steadies the bit within the mouth, and it controls the lever action of the reins. The curb chain acts as a fulcrum pushing the bit down onto the horse's bars, amplifying the bit's pressure directly onto the sensitive bars of the horse's mouth.
A bit with no curb chain gives no leverage to the reins.
If the chain is too loose, it allows the bit to rotate in the horse's mouth too much. If it is too tight, it applies pressure from the reins immediately, giving the horse no time to respond to rein instructions before uncomfortable pressure is exerted on his mouth. Even more painful and damaging results can be caused by a curb chain that is too tight.
Common practice is to adjust the chain so that it comes into action only when the shank rotates about 45 degrees back.