Flail chest describes a situation in which a portion of the rib cage is separated from the rest of the chest wall, usually due to a severe blunt trauma, such as a serious fall or a car accident.he Mechanics of Flail Chest. For a person to draw a breath, the muscles around the rib cage and the diaphragm have to move to expand the chest cavity. This creates a vacuum that is filled as air enters the lungs. If this expansion is hindered, the ability to draw air into the lungs is diminished.