HIT trains on the erroneous assumption that momentary muscular fatigue is a good way to grow muscle. So they only have you do one or few sets because as you get tired the intensity is unproductive. So HIT will have you squeeze it out until you drop, and then if you get 1 more rep the next time, supposedly you're growing.
oh and you wait a while in between workouts, which is bad.
pros - it's really simple and sounds sort of scienc-y
cons - it isn't really based on anything real, and isn't that effective