Regular sneezing is a reflex that rapidly expels air and mucus through the nasal cavity in response to some kind of irritation or foreign body. Reverse sneezing has a similar effect, but it is the rapid inhalation of air rather than exhalation. Also known as paroxysmal respiration or pharyngeal gag reflex, reverse sneezing is a sudden, rapid and extreme forceful inhalation of air through the nose causing the dog to make repeated snorting noises which often appear as if the dog is choking.
Reverse sneezing is not a sneeze at all. It is a spasm that occurs when the soft palate and throat become irritated. Aptly named, this behavior can appear much more troubling than it actually is, and generally isn't cause for alarm.
Reverse sneezing is characterized by a backward head motion, a closed mouth and lip sucking in. Gagging usually causes the dog to swallow after extending its neck and opening its mouth.
Boxers, Shih Tzus and dogs with flat faces have a soft palate that is stretched out more, and they can have bouts of reverse sneezing more than other breeds because they can actually suck the palate into their throat when they inhale. Smaller breeds are also more apt to be affected because they have a smaller throat. A genetic factor is suspected to be involved with these smaller breeds.