When you have the sensation of being full up to the neck retain the air for a few seconds and then very slowly send it out in little puffs again.
This is a splendid exercise, but I want to warn you not to practice any breathing exercise to such an extent that you make your heart beat fast or feel like strangling.
Overexercising the lungs is as bad as not exercising them enough and the results are often harmful.
Like everything else in singing, you want to learn this gradually. Never neglect it, because it is the very foundation of your art. But don't try to develop a diaphragm expansion of five inches in two weeks.
Indeed, it is not the expansion that you are working for.
I have noticed this one peculiarity about young singers—if they have an enormous development of the diaphragm they think they should be able to sing, no matter what happens. A girl came to see me once whose figure was really entirely out of proportion, the lower part of the lungs having been pressed out quite beyond even artistic lines.
"You see, madam," she exclaimed, "I have studied breathing. Why, I have such a strong diaphragm I can move the piano with it!" And she did go right up to my piano and, pushing on this strong diaphragm of hers, moved the piano a fraction of an inch from its place.
I was quite aghast. I had never met such an athletic singer. When I asked her to let me hear her voice, however, a tiny stream of contralto sound issued from those powerful lungs.