During a spasm, the diaphragm doesn’t rise back up after exhalation. This inflates the lungs, causing the diaphragm to tighten. This can also cause a cramping sensation in the chest.

Exhalation [n.] - The act or process of exhaling, or sending forth in the form of steam or vapor; evaporation. - That which is exhaled, or which rises in the form of vapor, fume, or steam; effluvium; emanation; as, exhalations from the earth or flowers, decaying matter, etc.

Ribs 11 and 12 are considered floating, vertebral, or free, ribs because they terminate in the posterior abdominal musculature and do not attach to the sternum. The costal cartilage of ribs 1 through 10 facilitates the elastic recoil of the thoracic wall which contributes to passive exhalation.