Horse "hanging out" and not bending when lunging?

im not a big fan of so called training aids but im a huge fan of the pessoa. if you havent heard of it its an amozing tool for lunging. it develops topline, bend and correct action from behind. it uses a system of pulleys to encourage the horse to work through from behind in a correct way. if he tries to evade it places pressure on certain areas to encourage it to work properly again. they are a fantastic training aid and i highly reccommend them