This video is a very scientifically grounded yet easily understood review of how short bouts of interval training can achieve dramatic improvements in your fitness level (VO2Max). Great effort by the Australian Broadcasting Corp!
In brief, high intensity interval training increases the mitochondrial density of your muscle cells. These are the energy producing elements of your muscle — more you have, the more oxygen your muscles can uptake to burn glucose as fuel.
Low mitochondrial density not only makes you less fit but also exposes you to a range of health risks.