Multiple mechanisms of action, many things going on. I read a couple of months ago, or saw on a YouTube, clinical studies on early cases that having the patient prone on a bed with the head slightly downhill, improved survival by helping the junk to drain out of the alveoli. As far as increasing O2 delivery, if you have both the air sacs filled with mucus, and some % of the blood cells out of commission, then a higher O2 % would help the few remaining blood cells, when they *do* make it to somewhere they can pick up O2, load up with more oxygen. Osmosis, diffusion, mole fraction, yada yada.
Higher O2 is not always a good thing. >60% is hyperoxemic tx and promotes fibrosis via free radical injury. Proning does not improve 30 day survivability, although it does transiently improve oxygenation, so maybe its just enough to get someone on the fence through. Head down is a bad idea in prone or supine ventilation as it causes an increase in WEST Zone three via a restrictive mechanism (abdominal contents reducing thoracic volume). So the notion of draining is not founded.