Breathing

A good breathing pattern is fundamental to the use of the upper body. Shallow breathing causes tension in the neck and pulls the shoulders up and back, disturbing general placement. The sideways expansion of the ribs allow an efficient and deeper use of the lungs without affecting postural control, and therefore discourages tension. Breathing well feeds the hardworking muscles and increases stamina in allegro. It is also used to phrase movement, and to help with the lift required to pirouettes and jumping. Controlled rhythmical breathing necessary in all forms of exercise to focus the mind and instill confidence.
Source : THE FOUNDATIONS OF CLASSICAL BALLET TECHNIQUE ROYAL ACADEMY OF DANCE