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The curb chain applies pressure to the curb groove under a horse's chin when the curb rein of the bit is used. When the curb rein is pulled, the shank of the bit rotates back towards the chest of the horse and the cheek (upper shank) of the bit rotates forward (since it is a lever arm).f used with a loose curb chain, it allows the shanks to rotate more before the curb chain is tight enough to act as a fulcrum and exert pressure. This extra rotation can warn the horse before pressure is exerted on his mouth, so he may respond beforehand.
The Tom Thumb and any other bit with shanks are curb bits, which use a leverage action. So, if the bit has shanks, it is always a curb bit, and never a snaffle. The shanks on a curb bit do not have to be long to put some pressure over the top of the horseâs head, and behind the chin. That is how a curb bit works. So even a bit with two or three-inch shanks is a curb bit.