Collection is a natural attitude of mind and body of the horse. When at its leisure, the horse will move with its body naturally relaxed. When the horse senses a challenge of some sort, it will prepare itself for action both by greater arousal of attention, and also by collecting its body. That means to draw the body in upon itself so that it becomes like a giant spring whose stored energy can be reclaimed and directed against the outside world by releasing whatever has held it under compression. The largest organic spring in the horse's body, and therefore the easiest one to observe in action, is composed of the spine and the associated musculature that draws it together in much the same way that a bow is drawn by an archer.
When a strange horse comes upon the scene, when a horse catches the scent of a predator, or when a horse scents a potential mate, the attention of the horse is naturally aroused and its body prepares for action. It raises its head, arches its back, and in so doing brings its legs more under its body where they can maintain the bending of the spine by their contact with the earth. With each step forward this spring is released slightly and then immediately recompressed.
Collection of the horse's body has two readily apparent benefits. The first is that if danger suddenly appears the horse has a large store of energy ready for instantaneous release and so it is much easier for it to spring away from the danger than if it had been caught flat-footed. The second is that the horse is properly prepared to perform movements like the levade. The levade is superficially like the motion in which the horse rears up. A rearing horse is very dangerous for its rider because it is an unbalanced movement and the horse must either fall foward to the ground or fall over backwards on its rider. In the levade, however, the horse's rear legs are well under it, and it can safely support itself in an upright position for a time and then lower itself to the ground under control.
Through training, the horse learns to collect itself when requested to do so by the rider. The observer receives the impression of great strength held under perfect control A horse doing a collected canter can, still under perfect control, move into an extended gallop like an arrow being released from a drawn bow.
