The curb uses a leverage action, since it attaches to the cheek and headpieces of the bridle at a point above the mouthpiece, and the reins attach to the end of a shank (essentially a lever). When the rein is engaged, this lever action puts a level of pressure on the poll through the cheek and headpiece, and on the mouth and jaw through essentially squeezing the lower jaw between the mouthpiece and the curb chain running under the jaw.
eBay