HIT stands for High Intensity Training for bodybuilding and fitness
The man who introduced this HIT method and brought it’s fame was Arthur Jones, a difficult man with a very distinctive character. He is also the inventor of the Nautilus exercise equipment, which can be seen as the basis of almost all current gym equipment. He made a fortune with both. Famous Bodybuilders who have used his method are the Mentzer brothers and later on Dorian Yates among others.
Jones (you can see him here) assumed that many bodybuilders train too often and do too much in a workout at a too low intensity. This does make bodybuilders chronically over trained, their bodies don't get time enough to really recover. With this he was certainly making a point, training is for many people simply more or less addictive. But we often forget that not the training makes us better/stronger but the recovery form a workout does. And recovery takes time; a hard workout will take more then a few days to fully recover from. For Jones, this meant that training should be limited to just a few work outs a week, no more.
He also thought that you had to focus mainly on the large, complex / compound exercises in order to get a maximum of muscle working at the same time; this to get a maximal hormonal response.
Furthermore, he believed in very little rest during a training session, the whole HIT workout should be carried out without long pauses. After a very short rest, up to the next machine and go again.
Another point for Jones was not looking so much at the weight you use, but to look at TUL (time under load), how long are the muscles working during a work out is what matters. For this he used a slow way of doing the exercises, 3 seconds up, 1 second pause, 3 seconds back, 1 second pause, etc, and go on until you are absolutely not able to continue anymore. And that should be taken literally; you really have to push a full 100%. Just that last bit, according to Jones gives the real training stimulus.
How did Jones come to his method? (More about the man here). He himself was a little man who had difficulty to build extra muscle. Looking at big strong animals, especially lions, brought him to an idea. In his eyes they did little: a lot of sleeping and relaxing with short bursts of work, when they went for hunting. This principle is what he worked out in his HIT method, but with his difficult character he made IT a very dogmatic / rigid total which is very difficult to follow. But that certainly doesn't mean that his method hasn't got valuable elements in it, but we should look at it with a bit of flexibility.
Looking at a few simple basics we know the following:
1 - Physical training is disturbing the existing balance, this disturbance forces the body to adapt. That certainly meets HIT. And the fitter you are, the harder the training stimulus must be to upset the existing balance.
2- Principle of super compensation: after a training stimulus, the next stimulus should followed at the right time. So it's only after a recovery period before you give a hard stimulus again.
3 - 80/20% rule: 80% of the possible outcome can already be reached in 20% percent of the time you would need for maximum results. If you are training in the HIT method, you train short and infrequent. The principle also fit's in in HIT.
4 - Training is specific: if you training you have to be sure that what you are doing is useful. Bodybuilders / fitness people want bigger muscle / become stronger. To reach this you must train hard and heavy, training hard takes long recovery. This also fits in.
Some things I do not agree with: doing a proper Warm up is always useful and makes you perform better and gives you less risk off injury. Jones was wrong in this, he says that warming up too much will hinder your training/progress. That is if you’re doing hard training. I do think that training more often is not harmful; this training simply should be easy and help recovery.
HIT training in short, train with maximum intensity, short train, train not too often, only train the large and complex exercises. An addition from me: this method is very stressful, always make sure to warm up proper, and be careful with pains, maximum loads gives quick injuries.