The Full Port de Bras involves raising the arms from bras bas to 1^st position to 5^th position, and lowering them through 2nd position to bras bas. The arm movement comes from the middle of the back, never disrupting basic posture. With a slight inhalation to lengthen the torso upward, the chest and upper arms initiate the lift of the arms as the shoulder blades draw gently downward along the back. When the arms are raised above shoulder height in any port de bras, this counter-pull is especially important. There should be full control of the back: the wide muscles below the shoulder blades are engaged to pull the shoulders down toward the centre of the waist, with the shoulder line remaining open. The abdominal muscles are strongly activated to stabilize the waist and counteract the tendency to throw shoulders backward, which distorts the long, compact line of the torso and pulls the head out of alignment.

Source : THE FOUNDATIONS OF CLASSICAL BALLET TECHNIQUE ROYAL ACADEMY OF DANCE