Pneumothorax
Air enters the pleural space and abolishes the negative pressure that normally holds the lung expanded against the chest wall, so the lung collapses inward under its own elastic recoil, and if the air entry becomes one-way the resulting pressure buildup can compress the heart and great vessels, a time-critical emergency.
First principles
The lung stays inflated because of negative pleural pressure, not muscle
Normally the pleural space holds a small negative pressure, generated by the opposing pull of the lung's elastic recoil (inward) and the chest wall's tendency to spring outward, which sucks the lung wall against the chest wall like two plates of glass held together by a thin film of fluid. If air enters the pleural space, through a breach in the visceral pleura (spontaneous, often from a ruptured subpleural bleb) or the chest wall (trauma, iatrogenic), this negative pressure is lost, the two surfaces separate, and the lung's own elastic recoil causes it to collapse inward, independent of any muscular effort by the patient.
Educational content pending clinical review. Not medical advice.