A 32-year-old man presents to the trauma bay following a high-speed motor vehicle collision with multiple rib fractures (ribs 4–8 on the left). Chest X-ray shows a collapsed left lung with mediastinal shift. A second trauma patient, age 28, has similar rib fractures but presents with severe respiratory distress, paradoxical chest wall movement, and hypoxia despite clear lung fields on imaging. Which clinical feature best distinguishes a hemothorax from a flail chest?