Interval training has been used by top physical trainers and athletes for quite a long time now.
The main "science" behind interval training says that by mixing short bursts of high intensity work with lower intensity stretches that allow the body to cool down somewhat, you're simultaneously overloading the aerobic and anaerobic systems.
With interval training, you thus get the benefits of aerobic and anaerobic training at one and the same time.
There are other benefits of interval training, too. It's more difficult to become injured through this practice, because the high-intensity anaerobic periods don't last that long and the body gets sufficient recovery time between them, and you are listening to your body more acutely.
There is also less chance of developing nerve-wracking (even though usually harmless) conditions like heart arrhythmia because during the aerobic cooldown you are still working out, keeping your heart rate elevated and the blood oxygenated, so that the body remains "used to" being physically active above the norm.
Finally, for people who tend to get bored with working out and so drop their training regimen, interval training is more mentally stimulating because of its constant change-up...and, they can maximize their health benefits in a shorter workout.
HOW TO BEGIN WITH INTERVAL TRAINING
You begin a day of interval training the same way you begin any other workout day. You begin your workout slowly, gradually building up to a more intense level to allow your muscles and heart to get warmed up.
Using a heart-rate monitor or by closely checking your pulse for fifteen seconds at a time, try to keep your heart at an elevated but not 100% of your normal maximized increased rate (about 70% is good) for five minutes. Once you have achieved this plateau, you are warmed up enough to go for a burst of high-intensity activity.
This "high intensity" is relative to what you have been doing for the last five minutes and your level of physical fitness, just like your heart rate is. If you have been jogging, launch into a "check out", that is pick up the pace so that you are running at a long-distance race pace. If you have been running, break into a full-on sprint, going as fast as you possibly can.
Now, don't worry that you won't be able to maintain this full-on 100% level for very long; you aren't supposed to. The whole point of this anaerobic burst is to wear out and even break down your muscles so that they are forced to eliminate toxins such as lactic acid (the "burn" that you feel), open up your lungs to their total capacity (which eventually opens up even more alveoli, the tiny web-like network of air sacs), and make your heart swell to allow maximum blood flow.
Your muscles will quickly become much less flexible, and won't be able to contract well. Remember: this is normal and is the very effect you are going for.
Pay close attention to your body as you are "bursting". As soon as you feel you have pushed yourself to the limit and you have "hit a wall", gradually slow down. Do NOT stop or just walk; this will lose much of the anaerobic benefit and "psyche you out" (and, although this is VERY UNLIKELY to happen, it's possible to induce a heart attack by suddenly going from 100 to near 0). But gradually slow yourself back down to the pace you were maintaining just before your burst.
Your body once again becomes able to efficiently exchange carbon dioxide and oxygen, and it will begin rapidly feeding nutrients to your depleted muscles to repair them (the breaking down and re-building of muscles is actually what causes them to become leaner and stronger; you have to go through Hell to get to Heaven, as they say).
Do this pattern repeatedly for at least 30 minutes.
KEEP ON KEEPING ON
As you continue your interval training, you'll find yourself able to sustain your 100% bursts longer; your speed during these times will increase; and even your aerobic pace will be faster and you'll be able (and wanting to) sustain it even longer than 30 minutes. Let this happen and push yourself to new heights over time, but BE CAREFUL. Listen very closely to your body and make sure you eat highly nutritious foods, stretch well, and remain hydrated.
Pushing yourself to these extremes is, frankly, potentially dangerous. But if you build up gradually and take care of yourself you have little to worry about.
Interval training breaks down and rebuilds your muscles so that they are burning calories at a high level even when you are asleep. Your metabolism increases. Your breathing gets better and your blood is more oxygenated at all times, so you have more energy and mental benefits like increased concentration.
You feel more self-confident. And, your workouts are not monotonous. Your muscle tone increases dramatically and you get a sleek, well-moving body.