If you body is out of position at address, making a sound backswing will be nearly impossible.
Posture is the key to a correct setup.
This curving of the spine prevents a full and even turn. The hands, arms and shoulders need to move in sync. If you keep your backbone straight it will become the axis around which your shoulders, arms and hands turn. It is important to maintain your spine angle through the swing. Bobbing your head up or down or moving your back changes the plane on which your arms and shoulders move and will destroy your swing path.