“If collection is defined not merely as the bodily state in which the horse is best and most easily able to comply with the schooling requirements of its rider, it signifies, when applied to all the branches of horsemanship, increased attentiveness and a readiness of execution that best corresponds to the particular object in view.
In this sense collection is a relative term, comprising the readiness of the race horse at the start as well as that of the manege horse, flexing the springs of its hindquarters in the levade for the most difficult manege leap, the capriole. In both cases it involves the horse’s positive control of its own body, as manifested by the fact that the time between command and execution of a movement is as brief as possible. The length of this “reaction time” will depend upon the precision of the controls, which do not assault the horse and confuse it but are clearly understood by it, upon the controlled activity of the trained nerves that command the movement, and upon the training and the strength of the muscles and their ability to execute what is required appropriately.”
Waldemar Seunig, Horsemanship