Perhaps when the PPV enters the tube it inflates the cuff, but during the expiratory phase it relaxes so that there is less chance of barotrauma to the tissues?
Had an instance like that a few weeks ago. Guy rolled his car and wandered off in a daze. When his head stopped ringing he called 911 but was lost in the wilderness. Searchers went out to look but he was finally found by an unmanned aerial surveillance platform with FLIR.