Prevents the curb bit from rotating too far in the horses mouth As the reins on curb bit are applied the curb chain comes into action and puts pressure on the chin The chain must sit flat to avoid uneven pressure and be fitted correctly to prevent the bit from rotating over 45 degrees in the mouth Prevents the curb bit from rotating too far in the horses mouth. As the reins on curb bit are applied the curb chain comes into action and puts pressure on the chin. The chain must sit flat to avoid uneven pressure and be fitted correctly to prevent the bit from rotating over 45degrees in the mouth.