The double bride is primarily used in the higher levels of dressage when both the horse and rider are ready to communicate though very refined aids. The horse literally carries two bits in its mouth, each of which is connected to separate cheek pieces so that they act independently from each other.
The bradoon, which is a form of light snaffle, is placed slightly higher in the mouth (closer to the corners of the lips), while the curb (often a Weymouth) rests slightly lower in the mouth. Each bit has its own separate set of reins, which the rider carries between different fingers (typically the snaffle rein goes under the fourth finger, as usual, and the curb rein goes under the third finger, but there are many variations).
The function of the bradoon is to work as an ordinary snaffle would – applying pressure on the lips, bars, and tongue to exert directional control – and particularly to keep the elevation of the head. The function of the curb is to help increase longitudinal flexion at the poll, encouraging throughness and self-carriage in the advanced horse.