A curb bit works on several parts of a horse's head and mouth. The bit mouthpiece acts on the bars, tongue and roof of the mouth. The shanks add leverage and place pressure on the poll via the crownpiece of the bridle, to the chin groove via the curb chain, and, especially with a "loose jaw" shank, may act on the sides of the mouth and jaw.
The curb bit's mouthpiece controls the pressure on the tongue, roof of the mouth, and bars. A mullen mouth places even pressure on the bars and tongue. A port places more pressure on the bars, but provides room for the tongue. A high port may act on the roof of the mouth as it touches. Some western style curbs, particularly the Spade Bit, have both a straight bar mouthpiece and a high welded port., thus acting on the bars, tongue and palate. In the wrong hands, such bits can be extremely severe, but on an exquisitely trained animal, they allow the rider to communicate with the horse with a simple touch of the fingertips to the reins.
Curb bits can also be purchased with a variety of jointed mouthpieces that are sometimes mistakenly called "snaffles", some of which (like the twisted wire) can further increase severity. Jointed mouthpieces increase the pressure on the bars due to the nutcracker action of the mouthpiece. In addition, the joint angle is altered by the shank leverate to tip the bit downward and into the tongue. These bits, sometimes called "cowboy snaffles" due to their popularity among western riders, are actually more harsh than a curb with a simple, solid, ported mouthpiece.
Curbs are generally placed lower down in a horse's mouth than snaffle bits, touching the corners of the mouth, or creating a single slight wrinkle in the lips. The lower the bit is placed, the more severe it is as the bars of the mouth get thinner and so pressure is more concentrated.
The curb chain should be adjusted correctly, lying flat against the chin groove and only coming into action against the jaw when the shank is rotated, but not so loose that the shank exceeds 45 degrees of rotation.