Now when you engage the horse and ask for vertical flexion, an interesting thing happens. As the horse releases to the pressure of the bit by flexing vertically at the poll, the topline of the horse tightens, contracting the neck and back muscles. I use the analogy of the ready position that a shortstop in baseball assumes to scoop up a groundball. By collecting the horse you're helping it to balance and carry the rider, which is not exactly an easy thing for a horse to learn when packing an additional 150 to 250 pounds of tack and rider. There are two main categories that bits fall into, snaffle bits and leverage bits.
First lets start with how to define a snaffle.
A snaffle is designed so that the reins have direct contact with the horses mouth. A snaffle does not have a shank and applies no leverage. Most snaffle mouthpieces are jointed in the middle and work much like a nutcracker would. Some snaffles are made with a solid mouthpiece but these bits are primarily used for driving horses in harness. The primary pressure point of the snaffle is the tongue followed by the bars of the jaw and then the corner of the mouth.