During inspiration, the diaphragm contracts and moves downward, increasing the volume of the thoracic cavity. This causes the intrapleural pressure to decrease (become more negative) relative to atmospheric pressure. The negative intrapleural pressure creates a suction effect that pulls the lungs outward, causing them to expand and air to flow in. This mechanism is essential for effective ventilation and gas exchange. An increased intrapleural pressure would compress the lungs, impeding expansion and breathing.

Reference: Guyton and Hall Textbook of Medical Physiology, John E. Hall, 14th Edition.