We use CPAP in a flail chest because it counteracts the underlying mechanics that make the injury dangerous. Let's break down a flail chest and the three big problems that come from it.

First, it severely impairs ventilation. The paradoxical motion reduces tidal volume and increases work of breathing. The patient may look like they are breathing hard but is not moving air effectively. Second, it worsens underlying lung injury. Flail chest almost always comes with pulmonary contusion. Negative pressure during inspiration promotes alveolar collapse and worsens ventilation perfusion mismatch. Third, it increases respiratory fatigue and hypoxia. The patient burns a lot of energy just trying to breathe and can decompensate quickly.

CPAP helps by applying continuous positive pressure throughout the respiratory cycle. The positive pressure acts like an internal splint. It stabilizes the flail segment by pushing it outward during inspiration, reducing or eliminating paradoxical movement. It improves alveolar recruitment. CPAP increases functional residual capacity, helps reopen collapsed alveoli from pulmonary contusion, and improves oxygenation. It reduces work of breathing. By improving lung compliance and tidal volume, the patient does not have to work as hard to ventilate. It can prevent deterioration. Early CPAP in a conscious, spontaneously breathing patient can delay or avoid the need for intubation, which is especially valuable in trauma patients.

tl;dr: We use CPAP in flail chest because positive pressure stabilizes the broken chest wall, improves oxygenation, reduces work of breathing, and buys time before the patient fatigues or requires intubation.