Jathara Parivrtti - Belly Twist - works sides of neck, pecks, obliques and glutes. Everything is PASSIVELY lengthening in this pose. The lecture notes have a full list of everything happening in this pose. This pose relaxes the breathing and postural muscles because of gravity. Relax your abs on the exhale to release the lower back muscles. Engage the abs while breathing to mobilize the upper back. You can apply this breathing techniques in all twists.