A diaphragm is a partition; we often associate it with a membrane of some kind dividing one part, say, of a pump or filter from another part. Sometimes the pelvic floor is called the pelvic diaphragm but the diaphragm in the human body is the division going across the whole ‘tube’ of the body below the heart and lungs, and above the soft bag of the abdomen. This diaphragm has the oesophagus passing through it, taking food to the stomach, as well as major nerves and blood vessels. This diaphragm moves; it is a muscle, but a very strange muscle. It is attached to the inside of the lower ribs and the lower part of the breast-bone and to the sides of the front of the lower vertebrae. The fibres of the muscles all radiate in towards a tendon which is the centre, like the bull’s eye, of the circular partition which the diaphragm is. Imagine a dome, probably much nearer the top of your body than you might imagine. It is most domed before you breathe in, an important connection in your thinking, because the diaphragm is the muscle of breathing, not the only one but the main mover. So, I repeat, this domed partition, going across the body in a sense horizontally, is a muscle. It will be flattest after you have breathed in, the radiating muscle, as it contracts, drawing the domed central tendon area down. As this happens, the ribs also expand to the sides especially, and to the front somewhat. The thoracic cavity, in which the lungs and heart are found, becomes larger as the diaphragm contracts. The soft spongy lungs, which are attached to the inside of the thorax, are expanded by this movement of the walls caused by, principally, the diaphragm – and the air, through its inherent pressure, flows into the space within the lungs that has been created. In this basic picture there is no need for sucking in air, taking breaths, using the muscles around the shoulders. All these extra elements can be used, but need not be. The diaphragm together with the ribs and their associated muscles work together, in breathing in and breathing out, to maintain a rhythm which is using elastic stored energy in both phases. Volumes have been written on the part various elements play in this process, particularly the muscles of the abdominal wall (the muscles which you can use to pull your ‘tummy’ in) but I suggest you try to live simply with this rhythmical dome. An experienced conductor of choirs said to me recently that the best tonic for a long rehearsal is to get the whole choir to laugh. Laughter is infectious and it moves from one person’s diaphragm to the next. The diaphragm, uniquely in close contact with the heart in the human being, is the organ of empathetic responsiveness – which is just another way of describing breathing.
Acknowledge your diaphragm – then, go back to the head and the lengthening of your back, to the responsive support which your diaphragm can work from. Then let it get on with enabling you to laugh and to sing, to breathe and feel. With breathing, the first thing we need to learn is not to interfere.
Wiser Than We Know 
I’ve seen people who are very like-able and laugh to loud tend to sing better…….But one can also just laugh out to give a push to diaphragm singing