I've said it in the past posts but I'm not a fitness expert. I am a knowledgeable person when it comes to exercising with BW. One thing that I'm seeing a lot of lately on the Internet is confusion, doubt and disbelief about Visualized Resistance exercise (they're commonly called, Dynamic Visualized Resistance, or DVR's). I've seen some at the Atlas forums doubt that they work. I've seen a bunch of disgruntled guys splinter off into their own forum and utterly trash them and their number one advocate, John Peterson. Even on my beloved transformetric Forums there have been some questions about their effectiveness.
I can say without a doubt that properly done, they will work and work splendidly. The reason why people doubt that they do work is probably due to their lack of understanding as to how they work. A little kinesiology lesson is in the order. Let's take the upper arm for example. The biceps is the muscle at the front of the arm (anterior) which flexes the arm. One thing you have to understand about muscles is that they can only pull the muscle's two ends together. They cannot push them apart. So, to extend the arm, the triceps, on the back (posterior) of the arm, does this. Depending on the movement that you're doing, the biceps could be the initiating the movement, making it the agonist muscle. In that case, the triceps muscle would be the antagonist, relaxing and allowing the movement to occur. The triceps will also act as a brake to the biceps and that's what VR exercise takes advantage of.
We can use the antagonist muscle to exercise the agonist by tensing it simultaneously and powerfully. We don't contract them so hard that no movement occurs. that's an isometric. What we do is let just enough tension off the antagonist muscle to move the arm in the direction of the muscle that we want to exercise. The tension of the antagonist muscle gives us the resistance to develop the agonist muscle.
A question came up on transformetrics.com not too long ago about whether or not visualized resistance exercises could help with bone density. Many said no but this too is incorrect. Bone density is aided not by placing weight on the bone but from the muscular tension on the bone. Think of placing a gas can in the back of a pickup truck and using a ratchet strap to hold it in place. If you go too hard, you'll press the can so hard it will crush. Since both muscles of the particular bone are placing tension upon it rather than only one, the bone's density is greatly improved.
If this whole concept of antagonistic muscular tension building up strength in the muscles seems odd to you still, just remember this: Muscles get stronger by placing resistance upon them.
Muscle don't know and don't care how it got there. Anything that makes them tense powerfully will strengthen them be it your gravitational pull, a weight, or an antagonist muscle doing the job.