I've been looking into the science behind this for a while now... and the reason why there are so many reps is as follows:
The body has two types of muscles... fast-twitch muscles and slow-twitch muscles.
Fast-twitch muscles are the ones responsible for quick, powerful movements. They are designed for quick bursts of motion because they use energy less effectively, and therefore have to be bigger in order to work well. These are the large muscles of the body. And this is why people who want to build muscle usually do small numbers of reps with a lot of weight.
Slow-twitch muscles are the ones responsible for endurance, for repetitive motion with low resistance over a long period of time. They use energy more effectively, and thus can be much smaller because they don't need the same kind of power. These muscles are much more slim. So when you develop these, usually through endurance exercises which use a large number of reps with low weight, you develop your lean muscles.
Basically, look at a sprinter versus a marathon runner. Sprinters have very large muscles, because they need a lot of power fast. Marathon runners are built extremely skinny, to the point that a lot of them almost look malnourished. This is because their body mass is dominated by the smaller slow-twitch muscles.
So basically, what you need to do in order to cut the body's large muscles and slim it down, is to develop these endurance-specific slow-twitch muscles. This is why in the body feminization video, there are like 100 reps in some of the exercises. It's because you're basically trying to make your muscles tired not because of high resistance, but rather through persistent use. This means that your body is developing its leaner muscles.
Muscle development works through a continuous cycle of damage and then repair. That muscle soreness that you feel after exercise is actually finite amounts of damage. You build muscles by fatiguing them, and then your body repairs them. In order to do this muscle repair after a workout, your body needs protein. (Which is why bodybuilders take protein supplements.) If your body does not get this protein that it needs to repair the muscles through your diet, however, it will actually take it from other muscles to repair the muscles that have been damaged. So basically, doing these extremely-high-rep no-weight exercises will literally eat your body's big fast-twitch muscles away, and use that protein to rebuild the smaller, slimmer, lankier slow-twitch muscles.
That's the science behind this body-feminization video. The high-rep workouts are all about fatiguing the body's slow-twitch muscles, while the low-rep workouts like the butt-builder are typical resistance training to build up the bigger muscles on the body.
