Explanation
At the beginning of forced expiration Lung elastic recoil increases
During forced expiration, the muscles of respiration contract to actively decrease the volume of the thoracic cavity.
However, at the beginning of forced expiration, the lung elastic recoil, which is the natural tendency of the lungs to recoil inward, increases.
This recoil helps to push air out of the lungs by reducing the volume of the thoracic cavity.