parigha = an iron bar used for locking a gat
Rotation is automatic with side bending in the spine because of both the shape of the articular facets in the vertebrae and the spiral pathways of the muscles. To keep the action pure lateral flexion, the upper and lower ribs need to counterrotate in relation to each other. In this case, the upper ribs rotate posteriorly and the lower ribs rotate anteriorly. To achieve this, the internal obliques on the upper side and the external obliques on the lower side are recruited.
Also, if tightness exists in the outside of the standing leg hip joint (in the tensor fasciae latae, gluteus medius, or gluteus minimus), then that hip will try to flex rather than stay purely adducted. The standing leg should maintain hip extension (via the adductor magnus and hamstrings) to prevent this.
When there is tightness in the latissimus dorsi, lifting the arm overhead can push the rib cage forward (compressing the floating ribs and inhibiting breath in general) or pull the scapula downward even as the arm is lifting, potentially creating impingement of the biceps brachii tendon or supraspinatus at the acromion process.
With thanks to Lesile Kaminoff and the Yoga Anatomy book.