During silent breathing, expiration is normally a lifeless process and does not require muscles to work. When the lungs are strained and extended, stretch receptors within the alveoli send inhibitory nerve itches to the medulla oblongata, instigating it to stop sending indications to the rib cage and diaphragm to contract. The muscles of respiration and the lungs themselves are flexible, so when the diaphragm and intercostal muscles relax there is a flexible recoil, which creates a positive pressure, and air moves out of the lungs by flowing down its pressure gradient.
