Whitaker Training System.
The training system helps develop neck and back muscles. It encourages the horse to work in an outline which teaches self carriage. Adjustable roller - Adjustable ropes and pulleys - Sheepskin padded to reduce rubbing on the horse's back legs - One size only and fully adjustable to most horses or ponies - Designed to be used whilst lunging.
See here for a Whitaker Lunge Line to use with this system