Caruso and Tetrazzini on the Art of Singing
The audience, of course, thinks the prima donna's hand is raised to her heart, when, as a matter of fact, the prima donna, with a difficult bit of singing before her, is thinking of her technique and the foundation of that technique--breath control.
This feeling of singing against the chest with the weight of air pressing up against it is known as "breath support," and in Italian we have even a better word, "apoggio," which is breath prop. The diaphragm in English may be called the bellows of the lungs, but the apoggio is the deep breath regulated by the diaphragm.
The attack of the sound must come from the apoggio, or breath prop. In attacking the very highest notes it is essential, and no singer can really get the high notes or vocal flexibility or strength