The de Gogue was developed by the French horseman Rene de Gogue. He theorized that poorly or unschooled horses had three points of resistance: the poll, the mouth, and the base of the neck. The triangular system was designed to release that tension.
Fittings of the de Gogue
The de Gogue has two fittings: the independent and the command.
The Independent Fitting
This is used for longeing or free-schooling the horse, when the trainer is dismounted, and some trainers also begin early mounted schooling in the Gogue. The Gogue is made a leather piece with cords attached. These cords fork at the horse's chest and each run through one of the bit rings. The cord then follows the cheekpiece of the bridle up to a ring or pulley at the side of the browband, before going back down to snap to the leather piece near the chest. The leather extends so that it can attach to the girth.
The horse is therefore "in control" of the action of the Gogue: when he keeps his head in the acceptable position, the Gogue has no effect. When he sticks his nose out or raises his head, the Gogue comes into action, raising the bit in his mouth and applying slight pressure to the poll.
The Command Fitting
This is for use during mounted work. The leather piece of the Gogue is attached to the girth, and it forks near the chest into two cords. The cords are then run to the rings or pully at the browband, down the cheekpieces, and through the bit ring. From the bit ring they go toward the rider's hands, and snap onto shortened Gogue reins (which have metal rings at the end specifically for this purpose).
The rider should also ride with reins attached in the "normal" position to the bit, so he may use the Gogue rein as needed. Additionally, it can be jumped in (it has been used in competition) or ridden in cross-country. :)