I used to know a guy who trained (at sea level) for altitude events by doing his workouts breathing through a snorkel sort of thing.
It wasn't all *that* strange . . . just the mouthpiece and two flexible tubes (18 inches each, about 0.5 meter) which passed around behind his neck and were tied together at his back, with the ends of each open.
He didn't wear a face mask or anything.
The idea, as he explained it, was that each exhalation would fail to completely clear the tube, so that the following inhalation would include a mix of "outside" air and the re-breathed air from the tube (which had a higher CO2 content, and lower O2 content).
He claimed this helped him prepare for the lower oxygen levels at altitude.
I have no idea how well it worked, but he was something of an aerobic animal, and reasonably fit all-around.