Shipbuilder,
I know there are coaches that don't like the towel drill for whatever reason, but the drill has been around for quite some time.
Last spring my son was struggling on the mound. He was short arming his pitches and did not have much velocity either. He was pushing the ball rather than staying on top of it...
He began working with a respected pitching coach here in Arizona who was almost at his wits end trying to figure out how to break him of his short arming habit. He tried many different drills without success...he was still short arming the ball.
Finally, he went into his truck and pulled out a towel. He showed him the drill and as Sam did it over and over you could begin to see him get the idea of what the proper arm slot was for him. We used that towel drill many times before game and during practice to reinforce staying on top of the ball. This drill seemed to work for him.
Before 'discovering' the towel drill to establish proper (and I may add healthy) arm path Sam was maybe in the neighborhood of 65-70 mph on his pitches. Now, going on 7 to 8 months later, he is throwing much, much harder and much more accurately.
I'm not saying the towel drill is what did it, but I really believe it is what got him on the way to a better and healthier arm throwing technique where his natural ability and work could grow. Also we did focus a lot on his overall mechanics so in combination with the drill it has been a success. I would caution that as in any drill he warmup correctly and not jump into the drill full barrel to avoid either arm, shoulder or leg strains.
By the way, since Sam has improved so dramatically it has become increasingly dangerous to catch for him. I started wearing a catcher's mask, but after getting hit in the leg and foot numerous times because his pitches have increased in velocity as well in movement, I started putting on all the gear. I look kind of silly, but it's better than getting plinked at 80+ mph and rolling around on the ground trying to catch my breath while watching the knot grow on my knee...