Singing requires no powerful effort, but the point of support should fall where it is least apparent and least felt.

The attitude of fear, of uncertainty and doubt, namely the drawing in of the body below the waist, with the limp back, results in clavicular breath so laborious to control, because all effort is thrown upon the chest-muscles. How often we see a great artist when singing at his best, bear forward on the right foot, showing the straightened condition of the spine. And these are the voices that thrill with expression; these are the singers that carry an audience with them! There is no pretense to originality in this "secret" referred to in the interview with The Voice.

It was learned from Italian masters but has been corroborated in later years by reading more than once in different works of physiology that the straightened backbone frees the respiratory machinery.  

—Sabrina H. Dow. "Further Talk Upon Methods In Teaching Singing." The Voice, 1886: 165-166. 

The elongated or "straightened condition of the spine" does more than free the respiratory system. It is also an expression of an open ear, which enables the singer to sing.