Flexibility, in the Dressage Naturally system, comes after a few prerequisites. First the rider and horse need to have a healthy and happy partnership with the horse looking to the rider for leadership and wanting to please. This is achieved with ground training using Natural Horsemanship techniques. Second, the rider takes those NH techniques on the ground and carries them over to simple ridden work on a loose rein. The horse must stop, go, yield the quarters and shoulders, from the seat/body aids NOT the reins, in the walk, trot, and canter. Third, then the rider is ready to ride in connection with the reins since the reins will primarily be for feedback and placing the height of the neck. While riding in connection, the rider works on three things that lead to the "Sweet Spot" where everything seems easy. Relaxation, so the horse can melt to a stop at any time, alignment/balance using the basic alignment exercise so the horse is straight on the figure, and energy exercises so the horse is easily contained or goes forward from the seat. After all these prerequisites are done and the horse easily and willingly works in connection from seat/body aids then it is time to work on flexibility.
Flexibility exercises, in the DN system, work on lateral bending and longitudinal stretching and will lead to greater suppleness in the horse. Examples would be spiral in spiral out, figure eight and serpentine, change of direction with leg yielding, stretch to a working posture, and change of bend independent of direction.
The interesting thing about this system, which is set up a little differently than most dressage training scales, is that there is a built in success factor and there is a clear way to problem solve if you run into issues. The rider is already riding primarily off the seat aids way back in the loose rein "Freestyle" work so there is no speed control issue with the reins/hands. The horse is already pretty aligned and straighter with controlled energy through the "Sweet Spot" work, so is packaged and ready to work on the "Flexibilty" exercises.
I have found this way of thinking about the work super helpful. It takes classical dressage and slightly changes the perspective of what when, but it still is using the components of the training scale. All I can say, is Jet loves this progression. As I rehab him, we have worked at the walk all the way through the Flexibilty exercises, even touched on the Mobilty/Collectibility exercises, and now that he is trotting a bit we are working up to the Sweet Spot exercises at the trot. He is relaxed, balanced/aligned, listening about the level of energy, and correct in his carriage and frame. Loving this system.