Its not HIT, its HIIT. High Intensity Interval Training.
I don't see how it would help you with Penis Enlargement, its supposed to be used in Cardio to help burn fat quicker than regular training.
It works by just going as fast as you can for 30-60 seconds, than taking a minute of slower cardio, than speeding to maximum speed again for 30-60 seconds or however long you can hold out. Than slow down, and recover. The trick here is to never stop, always keep going and recover your breathe without stopping.
It has nothing to do with gaining muscle mass. Just helps you burn off fat cells quickly without long useless cardio sessions. I've used it before and its fun once you get used it its intensity.
The intervals can be also done with weight training to provide a more cardio type workout, but its not very good cause you can't properly exhaust the muscles if you aren't doing maximum weight each time.