Can ground driving be used to introduce lateral movements to a horse?

Ground driving is an excellent tool for introducing lateral movements, and many classical trainers consider it the ideal environment in which to teach leg yield, shoulder-in, and even the early stages of half-pass before those movements are asked for under saddle. The reason ground driving works so well for lateral work is that the handler can see the horse's entire body, position himself to create directional pressure from multiple angles, and feel immediately through the reins whether the horse is straight, bent correctly, or evading the movement. The leg yield is typically the first lateral movement introduced through ground driving. The handler positions himself slightly to the inside of the horse's travel direction, applies inside rein pressure to create a slight flexion of the poll away from the direction of travel, and then uses the outside rein and his own body position — stepping toward the horse's outside hip — to push the horse sideways and forward simultaneously. The horse learns that pressure from behind and to the side means move away laterally, which is the same concept he will later be asked to perform from a rider's leg. Shoulder-in can also be developed effectively through ground driving. The handler positions the horse slightly off the arena wall, creates inside bend with the inside rein, and uses the outside rein to prevent the outside shoulder from escaping while his body language encourages the hindquarters to remain on the original track. This creates the three-track movement that defines shoulder-in and begins developing the horse's ability to carry more weight on the inside hind leg. The key principle in all lateral work done through ground driving is that the handler's body position must substitute for what a rider's leg would provide. Positioning, energy, and subtle pressure with a dressage whip or training stick on the horse's side fill that role. Trainers who develop a feel for using their own movement and position as a driving aid can produce remarkably precise lateral work entirely from the ground, which then transfers to ridden work with very little additional explanation needed.