She had developed her breathing capacity, but when she sang she held her breath back.
I have noticed that a great many people do this, and it is one of the things that must be overcome in the very beginning of the study of singing.
Certain young singers take in an enormous breath, stiffening every muscle in order to hold the air, thus depriving their muscles of all elasticity.
They will then shut off the throat and let only the smallest fraction of air escape, just enough to make a sound. Too much inbreathing and too violent an effort at inhaling will not help the singer at all.
People have said that they cannot see when I breathe. Well, they certainly cannot say that I am ever short of breath even if I do try to breathe invisibly. When I breathe I scarcely draw my diaphragm in at all, but I feel the air fill my lungs and I feel my upper ribs expand.
In singing I always feel as if I were forcing my breath against my chest, and, just as in the exercises according to Delsarte you will find the chest leads in all physical movements, so in singing you should feel this firm support of the chest of the highest as well as the lowest notes.
I have seen pupils, trying to master the art of breathing, holding themselves as rigidly as drum majors.
Now this rigidity of the spinal column will in no way help you in the emission of tone, nor will it increase the breath control. In fact, I don't think it would even help you to stand up straight, although it would certainly give one a stiff appearance and one far removed from grace.
A singer should stand freely and easily and should feel as if the chest were leading, but should not feel constrained or stiff in any part of the ribs or lungs.
From the minute the singer starts to emit a tone the supply of breath must be emitted steadily from the chamber of air in the lungs. It must never be held back once.