Training Mask cuts down the amount of oxygen that gets to your lungs, and by doing so allows you to be better prepared for high intensity scenarios. If your lungs become acclimated to training with low oxygen levels, then your energy levels are increased during normal oxygen intake.
The pulmonary resistance provided by Training Mask strengthens your diaphragm and increases the surface area/elasticity of the alveoli. The mask can be used during running/cycling as well as circuit training or other workouts. Bottom line is Training Mask increases your lung capacity by making your lungs work 9x as hard to get sufficient oxygen.