Curb bits are leverage bits. They act on the horse's mouth, poll and chin in several ways.
The mouthpiece acts as a fulcrum. When contact is taken on the reins, which are attached to the lower end of the shank, the leverage action tightens the bit in the horses mouth, exerts downward pressure the crown piece of the bridle over the top of the horse's head, behind the ears and closes the curb chain under the chin, exerting pressure on the lower jaw.
The severity of the curb bit is partly dependent on the length of the shanks. The longer the shanks, the greater the pressure exerted on the mouth and poll.
Needless to say, curb bits should only be used by experienced riders with quiet hands. Otherwise they can easily turn into instruments of torture.
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