Why does breathing into a paper bag relieve hyperventilation

The fact of using a paper bag when going through hyperventilation is a widely known situation, the reason why this can have beneficial effects upon the one who practices it is because the objective is to re-breathe  the air that goes in the bag because this air that was already in your lungs has a higher concentration of cardone dioxide and helps the body feel better in an easier, calmer and controled way.