The air inside the lungs is expelled out of the body in this action and it is called Rechak in Yoga.
The pressure on the inside air is increased by reducing the volume of the lungs; this is effected by the 3 reverse actions as following.
1. The diaphragm is pushed up.
2. The ribs are pulled down.
3. The ribs are closed inside as the shutters of the windows are closed.
As the volume inside the lungs is reduced, the air pressure is increased and the air is forced out. As the air is forced out, the air pressure is reduced to atmospheric pressure. This completes the exhalation process.