The high-intensity interval training provides many of the fitness benefits of prolonged endurance training but in much less time. Supposedly, even a few minutes of training at an intensity approaching your maximum capacity produces molecular changes within muscles comparable to those of several hours of running or bike riding.
Interval training, though, requires intervals; the extremely intense activity must be intermingled with brief periods of recovery. In the program, this recovery is provided in part by a 10-second rest between exercises. But even more, it’s accomplished by alternating an exercise that emphasizes the large muscles in the upper body with those in the lower body. During the intermezzo, the unexercised muscles have a moment to, metaphorically, catch their breath, which makes the order of the exercises important. The exercises should be performed in rapid succession, allowing 30 seconds for each, while, throughout, the intensity hovers at about an 8 on a discomfort scale of 1 to 10.
Source: The New York Times