I have been wondering about this too. I worked with for a dressage trainer a couple of years ago and when starting horses, he religiously lunged his horses with side reins before every ride. The side reins were always VERY loose, to encourage the horse to stretch their neck down nice and low. I recently started using this method on my OTTB and have noticed a huge difference. He feels much more relaxed and not nearly as stiff as he used to be. However, I'm noticing he doesn't stretch OUT as much as I would like him too, so if anybody has any thoughts on this matter/knows how to improve his stretch I would love to hear about them!