I know this thread is old, but I wanted to throw in my experience.
2 years ago, I was being towed behind a boat tubing. I never fell off or anything, but when I got back on the boat my shoulder started to stuffen up on me. later that night that the pain was so bad, I couldn't even change myself. My arm was basically stuck to my side and if I moved it even a fraction of an inch I would hunch over in pain.
For the next week I basically lived on pain killers, but still the pain was so bad that I was I sweating all day trying to deal with the pain.
By the time I got back to the gym, I went from doing 120lb DB inclines, to not even being able to move 20lb DBs due to the pain. I was out of the gym for a good 6 months losing a lot of what I worked so hard for.
I spent hours at the doctors, Ultrasound, X-ray, and MRI which didn't show any significant damage. There was something wrong in there, but the surgeon didn't see anything he could go in there and fix.
My shoulder has been getting slightly better over time, but I have never been able to to extend my arms straight out the the side, bend my elbow 90 degrees, then rotate my arm. External rotation was out of the question, my should would clink, then lock on me.
After some time I read this thread by DC and I thought..... yah it may help, but I doubt it will do much in my case.
The frist time I tried it I did about 15 "reps". The second time I tried it I did about 10 "reps" and the last time 20. I wasn't too serious, but I wanted to try it out.
About one week later I was totally pain free.. SERIOUSLY. I had a nagging shoulder in jury that was bugging me for years and this is all it took. For the first time since that on day in July I have zero pain and zero discomfort.
Well, there is my 2 cents. This exercise gets a thumbs up from me
