The term originated with the burlap gunny sack used by many cow hands to gentle a bronc for ranch work. Actually, sacking out begins as early as the first few days of the young horse's life when he is initially touched by the human hand and a halter. Once the yearling has basic respect for a handler and a halter, the trainer should begin to formally build the horse's trust and tolerance for stress.
Stress is a demand for adaptation and is necessary for growth.
Horses must learn to accept the sights and sounds in man's world in order to be useful. A trainer should gradually increase a horse's ability to withstand various stresses so that when later confronted with them, the horse will be able to cope. The flight mechanism of self-preservation which the horse has exhibited since prehistoric times explains why the horse is wary of unusual motions, sounds, sights, smells, and unfamiliar objects touching him. The horse's instincts tell him to flee rather than try to fight or ask questions. Once a horse is in a state of panic, he will often ignore all past training associations even if they were well-learned, positive lessons.