Breath is an aspect of posture. Posture is an aspect of breath. In a single word: relax.
When we breathe, particularly if we breathe properly, our entire structure shifts. Our spine bends, our sacrum tucks, our ribs expand, our belly and waist expand, the muscles in our throat and pelvic floor contract, and even our hips and shoulders may make minor shifts.
On the other side of the coin, misalignments and chronic patterns of tension and weakness in our body structure may inhibit breath participation in certain areas of the body and permit too much expansion in other areas. Therefore, proper postural alignment plays a key role in efficient breathing.