To stretch the muscles lying between ribs and to increase mobility at the joints where the ribs meet the thoracic spine. (It also stretches side muscles of the leg and spine, the latissimus dorsi muscle, and the shoulder muscles.)
Be standing. Cross the leg of the same side to be stretched across the front of the standing leg. Raise the arm of the side to be stretched upward to the ceiling and tilt it over the opposite side. Place the other arm down and behind your back
Slowly and in a controlled manner, push the outstretched arm over toward the opposite side. Allow the trunk to tilt sideways with the arm until a stretch is felt in the lateral torso and back. Hold for 30 seconds, return to the starting position and repeat on the other side
To strengthen the stretch you can us your other hand to draw the arm further across, and use a deep inhalation at end of range.