When the cardinal directions have been learned, by placing her muzzle at each point in successive on-demand trials, it is time to move on to other body parts. Next in the series is the "shoulder", representing exactly that - the front shoulder. "Shoulder-left", and "Shoulder-right", instruct the horse to place her shoulder (left or right) onto the target. This generally requires at least one lateral step into the indicated direction.  And upon touching the target, the mission is accomplished.... "X!"