HIIT, or high-intensity interval training is a strategy in which you give maximum effort (80-90% of your max heart rate) through quick intense bursts of exercise followed by less intense recovery periods. The workout continues with alternating work and relief periods totaling 20 minutes. One of the most important things to remember while doing HIIT is never allowing your body to adjust to one intensity level. The key is to push through the high-intensity intervals and lower your heart-rate during the low-intensity intervals.
Super-efficient HIIT is the ideal workout for a busy schedule. Research shows you can achieve more progress in a mere 15 minutes of interval training (done three times a week) than the girl jogging on the treadmill for an hour.
When you work at a lower intensity, like jogging, your aerobic metabolism kicks in and uses oxygen to break down fats and protein for energy, but when you push to your max your body uses its anaerobic metabolism. Not only does it keep your heart rate up and burns more fat in less time, but the effect of all that intense exertion kicks your body's repair cycle into hyper-drive. That means you burn more fat and calories in the 24 hours after a HIIT workout than you do after, say, a steady-pace run.