Part of the effectiveness of high-intensity training is excessive post-exercise oxygen consumption (EPOC), also known as “afterburn.” This helps your body continue to expend energy and burn calories even after you stop working out.
As fitness expert Dan Trink explains in his new book, High-Intensity 300, after intense exercise — either resistance, cardiorespiratory training, or both — your body continues to take in more oxygen to help restore it to its balanced, resting state. So, as your body continues to repair and reset itself, you’ll continue using energy (i.e. burning calories). And, this process can last for hours after your workout (and that's only one aspect of the benefits of these short-but-tough workouts.)
But, as Trink explains in his book, not every workout has a big afterburn; high-intensity training requires maximal effort, which creates larger oxygen deficiency/energy expenditure, and therefore a greater EPOC effect. Similarly, exercise physiologist Michele Olson, PhD, looked at a HIIT Tabata workout consisting of bodyweight jump squats. Using a metabolic cart, she measured energy expenditure before, during, and for 30 minutes after the workout. While the Tabata training session was only four-minutes long, it’s intense, and when you add the afterburn effect, she found that participants showed a similar caloric burn to that of a 20-minute brisk walk.
Of course you can't discredit the benefits of long, endurance-based workouts, particularly if you're training for a sport or race. But, if you're short on time, aim for EPOC.