When the lungs are full, millions of elastic fibres in the alveoli are stretched and the chest is higher than its relaxed position. So, to breath out, the intercostal muscles and diaphragm must simply relax and allow the chest weight and lung elasticity to pull the chest back to resting position. During forced breathing, when blowing out candles for instance, a special set of intercostal muscles acts opposite to the normal direction, and expels air very rapidly when they contract.
