It's capillary pressure!Its not inhaling, just sort of savoring the smoke, maybe the top of the throat, the gullet, for lack of a better word.
Seal one end of a tube, and the fluids (including smoke) naturally leak toward that end.
This enables you to enjoy flavor/Nicotine without having to be the Big Bad Wolf and huff and puff.
Then again, when you want to get that pipe fired up... puffing is needed.
But only briefly.
It's sort of like killing your enemies: there's one moment of three shots, two to the heart and one to the head, and then a long time of baking the corpse in a kiln until it is nothing more than fine white ash.