The third thing to look at is whether the bit has leverage. That means if you put 10 pounds of pull on the reins, the horse will feel 30 pounds of pressure squeezing his mouth. Leverage decreases the amount of time it takes for the horse to feel bit pressure. To make this kind of bit pressure understandable and horse logical you would have to soften the pressure to reward the horse three times as quickly as you would with a non-leverage bit. Because of this exaggerated pressure and release, curb bits impede true feel and understanding between you and your horse.
Curbs are also nondirectional.
Their pressure is felt as a clamping between the horse's chin and the bars of his mouth, and therefore can convey minimal direction to the horse. If you use a chain, the pressure is more noticeable underneath the chin. If you use a thick leather strap, the pressure is more noticeable on the bars of the mouth. In most cases, curb bits are used as a signaling device rather than as a training device to help the horse learn to shape himself correctly. One of the biggest mistakes everybody makes is picturing the bit by itself.