I had completely debilitating RSI where I could only use one hand at a time to type for 15m at a time before I had shooting pain all the way up my arm to my shoulder. His solution (which admittedly does sound like new-agey bullshit) took me from that to zero pain in 3 weeks. I have no idea if his syndrome exists or not, but his solution worked for me which is really all that matters.
Granted, I did not do an in-depth look into the evidence backing his arguments but I did find the argument of referred pain to be compelling, if not robustly proven. (e.g. the lack of existence of RSI in the time of typewriters, the increase in incidence of ulcers as soon as they were tied to stress by medical literature, physical back pain manifesting in some but not others with the same physical symptoms showing in MRI, etc)
1. My brain is capable of generating the sensation of pain in various parts of my body.
2. My brain has been conditioned to generate pain automatically (and unconsciously) in response to certain stimuli.
3. It is possible to recondition my brain to remove this automatic response.
The surrounding body of theories and recommendations are just methods for accepting these premises and completing the reconditioning process. For most people, accepting the "diagnosis" is the hardest part. The pain feels very real and very much caused by structural problems in the body. Personally, it was only after I had accumulated a sufficient set of evidence ("the nature and exact location of my pain is not consistent," "a nerve conduction study found nothing wrong," etc.) that I was able to begin reconditioning. The turning point came when, after I started to see some reduction in pain, I began experiencing strange symptoms in other parts of the body (e.g. headaches, which are rare for me). This aligned perfectly with what Sarno's book said would happen. After that, I was able to "buy in" 100% and the rest of my pain was gone within a few weeks.
I suggest reading Aaron Iba's blog post on the subject; it's what convinced me to give it a try: https://aaroniba.net/how-i-cured-my-rsi-pain
It had gotten to the point where I couldn't sit for 1 hour or pick my daughter up.
Pain is all I could think about waking up to before sleeping.
Then I read this comment on HN and my life changed. All I had to do is read Sarno's book and even before I could finish it I was feeling a lot better. It was like a decade of pain leave my body in 2 days. Though it does take time after that to learn how to get totally pain free.
I'm glad to say that someone else too read my comment on HN and posted that he was thinking of ending his life due to immense back pain but he too had a similar experience like me after reading the book and feels almost cured.
It's also not placebo either since I'm pain free for almost 3 years now.