The O2 trainer can increase inspiratory muscle endurance. This way you can increase your lung volume using less oxygen while working out.
While working out you wear the mouthpiece that comes with 10 caps to restrict the airflow into your mouth. You start with cap number 1 which has the largest hole of 13 mm and you work your way up. If you're going to train your lungs while at rest (not working out) then there are 4 caps for that. Bas Rutten (a former UFC Champ) designed this to be the ultimate in inspiratory muscle endurance.