In many disciplines, riders use long and low as a way to encourage their horses to stretch down and engage their backs. When done properly, this helps relax the horse, encourage more “swing” to its stride, and improves suppleness. The problem is, too many riders focus on head position and not enough think about engaging the horse’s hind end, riding the energy forward and asking their horse to lift his back by engaging his abdominal muscles. When you ride from the back to the front of the horse, you achieve the full stretch and have a horse that is balanced and light on its forehand.
Long and low happens when you have your horse on the bit and then you encourage him to stretch forward and down by moving your hands forward. The horse follows the contact into the stretch without becoming heavy (on the forehand). Contact must come first, then the stretch. Ideally, the horse’s poll should be no higher than its withers and your horse should feel like it is in arc — it’s head and neck are low, it’s back is rising up to meet the rider, and it’s haunches are lowered and stepping under its body.
Video demonstration of how to ride your horse long and low
The video below gives a very clear explanation and demonstration of how to get your horse to use it’s back and encourage it to stretch.