One of the questions I usually get asked is how to get the horse to stretch in the lunge without side reins. I will try to respond on that in this post.
First, I would just like to say that we use side reins. But we do not use them to make the horse bend his neck. We use them to teach the horse to seek contact on the bit.
When we use side reins they are initially so long that the horse can stretch all the way down to the ground. That is, the horse can lift his head as much as he wants. The horse, therefore, does not learn to stretch due to the side reins, but they teach the horse to seek contact on the bit.