During the ‘inhalation’ phase of the exercise, patients focus on directing the breath to the concave side of the trunk (inside of the curve) which helps expand those collapsed areas.

This is followed by the ‘exhalation’ phase, during which the patient isometrically contracts these respiratory muscles (your diaphragm and these deepest core muscles) by slowing the breath, eccentrically strengthening and stabilizing the associated muscles on both sides of the curve in this new position of trunk opening and spinal symmetry.

Practice by lying on your back, if possible, where your curve decreases by approximately 10 degrees. Or practice anytime while sitting at the edge of a chair, or in standing. Begin by tilting your pelvis forward and leaning slightly forward at your trunk.
In all positions, elongate upwards, like someone is pulling your head out of your shoulder girdle. Lift your belly button upwards. As you inhale and open your rib cage, hold your hands on your ribs or place a belt around your ribs to feel the outward pressure against it as you inhale and expand. Breath normal.
Next, slow your breathing down and sense the gentle rise and fall of your ribs as you inhale and exhale. Begin to take longer inhalations. As you inhale, there is a natural amount of extra expansion that occurs.
As you exhale, feel how your ribs can lower gently without fully shutting down/collapsing, feel how they can lower centrally as you elongate.
Practice as you slowly breath in and out.