The primary principle behind HIT is that training should be low volume.  Forget doing 10 sets of an exercise, or even 3 sets for that matter.  HIT proponents believe that only one set is sufficient for you to gain muscle.

This one set is typically done to complete failure and sometimes beyond.  This means that you do as many reps as possible with the weight, and then use some special technique to help push you to keep going.  This could be a training partner assisting on a few more reps, partial reps, or eccentric reps.

The idea is that with these techniques you can push your body out of its normal comfort zone and hit every muscle fiber with just one very difficult set.  Because of the intense nature of the program it is recommend that you only train each body part once per week.  This means that you are only doing one set per body part per week!

This efficient and time-saving structure is one of the main reasons for the popularity of HIT.  With this system just about anyone has the time to get in a productive training session.

But like any training program, your body will eventually adapt to it and your gains will begin to stagnate.  Because of this I recommend using HIT for 4-6 weeks at a time, especially after completing a higher volume program such as 10 x 10.

Remember that one training program is not the answer to building muscle.  Muscle growth is a long-term process that requires progression for months, not weeks.  Because of this you need a well-designed training system that allows you to continually progress and make great muscle gains along the way.