Nature Boy,
I'll probably screw this up a bit, because I'm a bit fuzzy on what I read a while ago about HIT (so correct me if I'm wrong HIT-ers), but essentially, HIT believes you should do each body part once a week, maybe twice if you do something like 2 workouts a week, full body each time.....You get in there, you completely blast yourself (failure on all sets) in a short amount of time, keeping the volume low, but the intensity super high and then you get the hell outta the gym and eat and recover. HIT believes that traditionally bodybuilders over train entirely too much, and they basically discount periodization and high volume approaches. They aren't saying that people can't have success on these types of systems...obviously, thousands of people have developed massive physiques on higher volume approaches, but they argue that high volume isn't the most efficient way of doing things. In fact, they say that you could achieve the same results with less time in the gym if you followed HIT principles and allowed your body to recover fully before going back in and blasting your muscles again.
I probably missed a lot and over-generalized, but I think that's pretty much the krux of what HIT says.
Anyone else care to add?