Refers to a
humongous "ah-choo!" that you consciously "divert" or "re-direct" the air-blast force of through your throat and out
your mouth --- rather than letting it travel its "standard" path through your sinuses and out your nose --- thereby reducing the risk of a "wet 'n' messy" spray from your nostrils.
Of course, you should still always cover your nose and mouth to minimize germy emanations in any case.