Summarize this article in one sentence.
Interval training is a form of training in which you alternate periods of moderate-intensity exercise with high-intensity exercise. Interval training may help increase fat loss, as it allows you to work out more efficiently. It also burns more calories — your body will continue to burn calories for hours after you have completed HIIT training. If you want to try interval training, decide on the length of your intervals.  Many people think interval training is complicated, but there are actually no prescriptive rules. You can choose any length for your intervals of intense activity. You can do them for up to two or three minutes, or for as short as 20 seconds. You may want to do some trial and error before setting hard and fast rules for yourself. You may find, when working out, two minute intervals wear you down fast. Instead, try going for intervals of one minute or 30 seconds. You can also alternate. You can warm up with a few intervals of 30 seconds, and then move on to intervals of a minute. Once you've decided on intervals, you can begin your routine. During that time, go at a steady pace for two to five minutes, then switch to a high-intensity pace for 30 seconds (or however long you choose). Continue this pattern for the duration of your workout routine.  Your HIIT workout should last between 20 and 30 minutes. Your heart rate should be at 85% during the high-intensity interval. When it comes to interval training targeted specifically at fat burning, rules depend heavily on your personal fitness level, target heart rate, current weight, and many other factors. Therefore, it's a good idea to discuss your desire to burn more fat with a personal trainer. He or she can help you figure out the time and duration of your intervals. Interval training comes with certain risks. You should be careful about engaging in interval training without talking to a doctor, especially if you have certain conditions.  If you have heart disease, you may want to talk to your doctor before attempting interval training. Injury is a risk if you rush into a routine that's too strenuous. It may be a good idea to start out with just one or two high intensity intervals during your regular workout.

Summary:
Decide on the length of your intervals. Alternate between a slow and steady pace and short bursts of intense exercise. Talk to a personal trainer. Consider risk factors.