There are several pieces of equipment or devices or gadgets out there designed to help riders put their horses in a more or less proper frame. The various riding disciplines favor different types of equipment. Dressage training equipment generally ranges from simple side reins to groups of devices known as martingales (running, standing, German), draw reins (which include all "running" or "sliding" reins), or the more complicated de Gogues and chambons. If the trainer rides or lunges over ground poles or cavaletti while using these devices, the horse can be encouraged to "use itself" while in this more or less proper frame.
The simpler and more direct the effect of the device, the more skilled the rider or trainer should be. Otherwise, the net effect will probably be negative rather than positive. The draw-type reins especially fall in that category. Even side reins are relatively easy to abuse. Most martingales are easier to use, but they still need to be adjusted properly and may not help as much as the rider may want them to.
Chambons are the most difficult and dangerous for the inexperienced horse person to use. Also, they should always be used for lungeing--not riding. There is a danger of the horse flipping over backwards. Of all the devices previously mentioned, the de Gogue is the most benign. It is easy to use and adjust. In it the horse is free to go long and low or short and elevated--even jump. Its beauty lies in its subtle complexities. Even your lunge line itself is versatile and can be used in several different ways as a kind of device.
The Pessoa system was invented by Nelson Pessoa (a great Brazilian jumper rider back in the day, as is his son, Rodrigo, now). It is an awesome addition to lunging equipment. It encourages a horse to not only flex at the poll, but also bring its hind legs under its body, thereby using itself properly. It can be adjusted from long and low to elevated and collected. It is truly an ingenious piece of equipment.
The bottom line, however, is that none of these devices can ride or train the horse by themselves, and they are only as effective and useful as the experience and ability of the rider or trainer using them. In the final analysis, devices only help accomplish what a skilled and proficient rider can do with a plain snaffle bridle and his or her hands, seat, and legs.
-- by Suzanne May