In patients with rib fractures, what benefit does grunting, or partial expiration against a closed glottis, provide?

A patient with rib fracture are at risk of atelectasis. Lung is a small organ that was attached to the chest. The lung normally forced to expand to chest size. In case of rib fracture, the lung might became collapsed because the chest might lost the force to make the lung to expand.
Grunting or partial expiration against a closed glottis might keep the lung from collapse as it makes the lung release the air slower. 

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