Stelling is a Dutch word, used to describe the bending of the head and neck to the left or right in relation to the spine. The lower jaw of the horse should move underneath the extensions of the atlas vertebra, the first vertebra in the neck of the horse. A proper stelling creates therefore a bigger space behind the lower jaw.
You can ask stelling bit by bit, while the horse should remain standing. If the horse takes the stelling and relaxes, you release pressure as well. You can then see that the stelling continues to work throughout the rest of the body in a bending of the spine, making the inner hip come forward.
Every horse has from nature a supple and a less supple side, because the muscles are uneven in length. To develop the horse symmetrically, you should ask the horse to walk circles to both left and right. You ask the horse to take some stelling, which makes the muscles on the outside lengthen and the muscles on the inside relax. At the same time, you ask the horse to bring his head forward down, to relax the back muscles and tighten the abdominal muscles.