This is only a mental exercise. In no way am I saying that this is the way it works. I'm just saying this is an interesting way of thinking about it.
Imagine that RSI is not caused by bad posture, lack of exercise, and repetitive motions alone. For example, there are many people with bad posture, who don't exercise, who use repetitive motions and do not have RSI (me, for example ;-) ). Imagine that in addition to these things a required trait is that the muscles are slightly tensed. Imagine that the tense muscles constrict blood flow. This causes an inflamation in the area and with repetitive motion (or even lack of motion) it generates an injury in the area.
Imagine that when you believe that you have solved the problem (by using a special keyboard, or changing the way you sit or changing the way you move) that your muscles relax just that little bit. Now the blood flows freely, there is no inflamation and the RSI goes away. You think it's due to your physical actions, but in reality you are cured only because you believe that what you did worked.
The idea behind the book (as far as I can tell -- I decided not to read it) is that conditions like RSI are real. The physical cause is real. However, the conditions necessary to cause the condition to occur are mental. It's a fairly compelling idea. RSI is a similar situation to the back pain that the author of the book studied. Many people suffer from it and the cure seems to differ widely -- if you can be cured at all. May of the cures don't really have convincing experimental evidence to show that it actually relieves any physical problems. The efficacy is based only on the apparent lack of new symptoms.
Now, I'm of two minds with this book. On the one had, like I said, the idea is compelling. The author believed that most chronic back pain's underlying cause was mental. The physical issues that in turn caused the pain, were in turn caused by the mental issue. The author apparently had considerable success in his practice, but I think it is important to note that his practice was for helping people who had back pain where no conventional treatment helped. So he's working from a point of sampling bias. While he may be correct that this occurs, it is hard to get an idea on how frequently it occurs.