Allison, I pulled a few things together from some of the information I have on hand about HIIT. You can read more about it from the link below - a web search on HIIT will also provide many interesting results
In research, HIIT has been shown to burn adipose tissue more effectively than low-intensity exercise-up to 50% more efficiently. In other words, HIIT speeds up your metabolism and keeps it revved up for some time after your workout. HIIT training burns a greater number of total calories than low-intensity training, and more calories burned equals more fat lost.
The reason low-effort theory of aerobic exercise came about is a study showed low-intensity exercise burns a greater percentage of calories from fat as opposed to carbs; however, when high-intensity exercise is practiced, the research showed the percentage of fuel from carbs is increased, while the amount of fat utilized is greater than or equal to that burned during low-intensity exercise.
Here's why using intervals of high intensity and moderate intensity work so well. The short-duration, high intensity stuff triggers adaptation (muscle gain, enzyme changes, cellular reorganization, lactate tolerance, cardiovascular improvement). You burn fat afterward in order to replace muscle glycogen. The longer duration, lower intensity aerobic activity (breathing deeply but still "conversational") allows you to create an energy demand that burns fat then and there. If you include both types, without overtraining, you're going to lose fat fast
The intensity you've experienced will have you burning a lot of calories for even hours after the workout - something you don't get from long, low intensity aerobics. This kind of interval training contributes greatly to your aerobic performance, and has also been found to improve the neurological pathways your body uses for recruiting muscle fibers.
Three or four HIIT sessions a week should produce significant fat-burning effects. To prevent overtraining, try to incorporate the program on your weight training "off days." Your HIIT program will likely be most effective if performed first thing in the morning on an empty stomach, but if you can't do it in the a.m., do it at noon or in the evening.
http://www.musclemedia.com/training/hiit.html
Nat