First of all, if you haven't yet, go to the Cyberpump page and see what they have to say about HIT. Read everything on it. Then come back and see if I have anything useful to say.
If you want a more in depth answer to this question, read the HIT FAQ on the Cyberpump page. In a few words HIT programs are weightlifting protocols that include very little work (usually less than four sets per bodypart) done very intensely (to muscular failure, or at least close to it) and very infrequently.
My brand of High Intensity Training is unique, but they all follow this general pattern. Obviously, most people do not train this way. Walk into almost any weight room in the country and you'll find countless people training with more sets, more exercises, and less intensity than I recommend. However, you will also see countless people wasting their time at best and suffering from chronic fatigue and immunodepression at worst.
High Intensity Training is the fastest, safest, surest, most efficient, and most easily measured system for achieving muscular growth. To explain why I will have to explain a little more about muscle growth.
Changes in muscle size are governed by two processes: Anabolism and catabolism. Anabolism means buildup, repair, or growth of the muscle, while catabolism means the breakdown of muscle. Both of these processes are continuous, and are normally well balanced. That is to say, most people create as much muscle, more or less, as they destroy.
Anabolism is an ongoing process in everyone. In sedentary individuals only enough anabolism occurs to offset (or almost offset) the normal catabolism. If this didn't happen, people would rapidly waste away whenever they got hungry or experienced any stress.
Resistance training, when done with enough intensity, stimulates an anabolic response. The body can be "tricked" into "thinking" that it needs to become stronger when it is stress at the limits of its strength. Only high intensity work will cause an anabolic response. If high volume, low intensity work stimulated anabolic responses long distance runners would have large quadriceps. They, in fact, do not.
Catabolism occurs in every person regardless of lifestyle or training level. We can see this because unused muscles, such as those immobilized by casts or those in comatose patients, atrophy. Catabolism is governed by various hormonal systems as well as by mechanical stress. In other words, elevated general stress levels (such as those experienced by people who are overworked) as well as specific mechanical stress (such as that inflicted by resistance training) are catabolic. That's right, weight training is catabolic, not anabolic. That's why resistance training often leads to muscle soreness - the soreness is an indication of muscle damage.
There are many ways to prevent catabolism. Eating frequently keeps blood sugar levels high, which keeps the body from cannibalizing muscle tissue for energy. Various drugs (so-called anabolic steroids among others) interfere with the body's natural hormonally regulated catabolic processes.
Obviously, any reduction in catabolism, if the rate of anabolism remains constant, will result in an increase in the rate of muscle growth. The rapid muscle gains experienced by steroid users show this.
It should be clear now that in order to maximize the rate of muscle growth it is necessary to train in such a way that the rate of anabolism exceeds the rate of catabolism by the greatest amount. We need to train in such a way that we can both stimulate a large anabolic response yet minimize the catabolic effects of training.
Remember, we need to operate near the body's strength limit in order to stimulate anabolism. We also need to minimize training volume, for training itself is catabolic. The best way to do that, for most natural bodybuilders, is to perfom one set per muscle group, done to failure. By working to muscular failure we are sure to tax our body's strength. By performing only one set we are doing our best to minimize exercise induced issue breakdown.
There are a large number of variables which affect muscle growth. In other columns I will relay more of what I have learned and figured out about optimizing muscle growth. I will explain what I believe is the safest way to train, what exercises to choose, what to eat, and what else we can do to achieve our goal of looking (and being) huge!