Before getting into the specifics of High Intensity Weight Training, a brief look into different muscle fiber types is necessary in order to understand what one wants to accomplish by lifting weights.
Although there are four muscle fiber types, classifying them as SLOW TWITCH and FAST TWITCH muscle fibers will suffice for our purposes.
Slow twitch fibers provide endurance and fast twitch fibers provide power/strength. If a weight is light or moderate, slow twitch fibers are recruited by the brain. However, if the weight is heavy enough, the brain will signal for the fast twitch fibers to join in.
To reap maximum benefits of resistance training one has to recruit and EXHAUST all muscle fiber types.