The principle behind the Elevation Training Mask is that if your body has less oxygen to work with, it will adapt to make the maximum use of the oxygen that it has available.
The Elevation Training Mask increases the resistance to air flowing in and out of your lungs. This increases the work of breathing makes it so there is less oxygen available and you have to work harder to get rid of the carbon dioxide you build up. This makes you go anaerobic sooner in the workout and at a lower power output. Just like weight vest training, once you have spent some time training in this mask, working out without it will be a breeze.