What Shoebox is saying is that the flexion needed for a bend starts in his ability to flex to the bridle, at the poll. So, if you horse is tight at the poll, he will be tight in his hindquarters, too. Bend requires him to be softer in the poll, flex to the inside a little bit, and be able to step under himself with his rear leg.
The exersizes Shoebox is talking about are a basic step to teach the horse to flex to the bridle, and to manipulate his head to help stretch out the muscles in the neck. I would add to that getting him to leg yield on a straight line, then to leg yield on a circle (spiral out).
look at a basic book of dressage foundational exersizes for some ideas on what to do.