For starters, there's the house itself. Research shows that sleeping at high altitude increases the production of oxygen-carrying red blood cells, which, when combined with intense, low-elevation workouts, dramatically improves athletic performance. Of course, it's logistically tricky to live high and train low - unless Nike makes you a special mock-altitude house. Which is exactly what happened. Molecular filters inside the house remove oxygen, creating the thin air found at 12,000 feet. Runners eat, sleep, watch TV, and play videogames at what their bodies think is high elevation. Meanwhile, they train at Portland's sea level.
Its actually quite easy to create a hypoxic chamber in your own house.
An industrial air compressor, a lot of gaffer tape to seal the room and you can sleep at "altitude" every night.