People who smoke may begin to develop emphysema, a homeostatic imbalance characterized by loss of alveolar elasticity. this makes expiration - normally a passive process - to become consciously an active process. why do emphysema patients need to exhale actively and how do they do that?
Naturally expiration is performed by the relaxation of the diaphragm, the internal intercostal muscles, and relaxation of the alveoli (loosing pressure within the lungs). If the alveoli do not relax and close naturally then air is not properly expelled from the lungs meaning you must put forth an active effort to expel the air. When you actively expire you consciously contract your internal intercostals and relax the diaphragm to force air out almost like a cough.