The postures of Hatha Yoga, by stretching the muscles and by toning, relaxing, and strengthening the muscles of the chest, diaphragm, and abdomen, and improving their mobility and elasticity, encourage deep and efficacious breathing.
The nature of many of these asanas is to trigger off naturally deep abdominal breathing. Stretching stimulates circulation and respiration, and by lifting bands of tension from the chest, diaphragm, and abdomen, sets free the muscles of deep breathing.
Smooth deep breathing is an integral part of the performance of many postures.
– pg 83, The Complete Yoga Book, James Hewitt