Like all wind instruments, the voice operates on a freely flowing and unforced stream of moving air. The larynx acts as a valve in regulating the flow of air through the vocal folds.
To ensure that the flow of air is unforced, and that the larynx is allowed to operate freely, the singer must breathe in a manner that lets the air move efficiently. The diaphragm, a very thin but very powerful muscle located directly beneath the lungs, flexes and relaxes, helping to cause the lungs to inflate and deflate.
The expansion and contraction of a blacksmith's bellows perfectly demonstrates this type of breathing motion.
One way to learn this technique is to lie flat on your back, with an object on your stomach, and observe the object as it rises and falls.