HIIT workouts are done very quickly. They're not at all like their steady-state counterparts in that most of them are completed in 20 minutes or less. This is still an effective exercise due to the fact that HIIT is done with such high intensity. The intensity is done at such a high level that you could not possibly force your body to perform any harder. Imagine you were alone in the forest and suddenly saw a cougar, imagine the intensity and the speed in which you would run in the opposite direction. That's the kind of intensity that's required to make HIIT an effective fat burning routine.
There are a number of different ways to do a HIIT workout. The first thing I'll go over is timing. The most common way to do a HIIT routine is 30 seconds of high intensity followed by 90 seconds of low intensity. This is not the only game in town though, and it can be adjusted depending on your goals and your level of fitness. HIIT is extremely difficult, and you may not be able to go right in and do 30 seconds of maximum intensity. It may be best to start with 15 seconds, or even 10 seconds, followed by 90 seconds of rest. The opposite might also be true. You may be very physically fit, and 30 seconds of maximum intensity might not be very challenging for you. If this is the case, then consider doing 45 seconds, or even 60 seconds of maximum intensity, followed by 90 seconds of low intensity. The rest period can also be shortened if you're in need of a more challenging workout. Shorten it from 90 seconds down to 75, or even 60 seconds, you'll definitely be challenged.
What's the best exercise to do HIIT with? As I'm sure you're well aware, there are many different forms of aerobic exercise. Rowing, the stair climber, the elliptical, the treadmill, running outside, the stationary bike, biking outside, swimming, cross-country skiing, etc. Is there a best exercise to do with HIIT? In my personal opinion, there is not really a best exercise. Any exercise, as long as you're putting your maximum effort into it, is going to be very beneficial when it comes to weight loss. That said, if I absolutely had to pick one form of exercise to do HIIT with, it would be running. You are absolutely guaranteed to melt fat off your body if you run at full intensity. Not all running is the same though, and believe it or not, running outside is not the same as running on a treadmill.