The diaphragm is a dome-shaped muscle separating the chest cavity (thorax) and the abdomen. The diaphragm is shaped like a parachute.
She grabbed at the diagram (diaphragm) and used it to build a parachute.
When you breathe in, your diaphragm contracts (tightens) and moves downwards until flat. At the same time, your intercostal muscles pull the ribs upwards and outwards, helping to enlarge the chest cavity.
This diagram shows clearly how parachute-shaped the diaphragm is.
When the diaphragm contracts, it flattens to make the chest cavity larger and air is drawn into the lungs. When it relaxes, air is expelled from the lungs.
Contracting and relaxing the diaphragm increases and decreases the size of your thorax.