This is a simple exercise and prepares the horse and rider for shoulder-in. It also develops suppleness, teaches control of the rein aids and places the horse from inside leg to outside rein and developing the on off aids. The exercise is hard to do well and requires consistent riding and aids.
This can be started when your horse is balanced round circle, responsive to your aids, and when you have good control of your leg aids and a balanced seat and soft hands
Start the exercise with a shallow bend and then gradually increase the bend as your horse's strength improves. Teach the exercise in walk to give you both time to understand it and then progress on to trot.
Over bending or horse drops to outside shoulder. Usually caused by the rider using too much inside rein. Keep your inside rein light and turn your shoulders slightly to the inside.
Your horse moves off track with insufficient bend. It may be that the inside rein is too tight, blocking the inside hind leg from moving, or that your horse has braces their ribs against your leg. Correct this by ensuring that you are correctly balanced, then close your body slightly towards your outside elbow.
If your horse becomes very contracted and short, check the balance of your rein aids and, if you are in walk, circle and stretch your horse softening the inside rein. If you are in trot, ride straight forward and ask for a little lengthening with a loose rein
Some horses have weak necks, and so their neck moves a lot. Ensure that your inside rein does not take back and support the movement delicately with your outside rein so they move from your inside leg to your outside rein.
Diane Followell Classical Dressage Trainer