This video is possibly the best example I've found of what Dr. Mike Marshall's arm action is supposed to look like. The catch is that the pitchers in the video aren't throwing baseballs. The pitchers are performing exercises from Dr. Marshall's wrist weights program.
4 of Dr. Marshall's students are shown here doing different things that include his wrong foot slingshot, wrong foot loaded slingshot, and pendulum swing wind-up drills.
One thing I've noticed repeatedly is that despite the straight-line acceleration of the heavy weights during these exercises, Dr. Marshall's students routinely demonstrate a dramatically different arm action when they throw baseballs.
The main difference is where the arm drives the baseball. In the weighted exercises (iron balls and wrist weights), his pitchers unquestionably apply force in a nearly straight line that results in extension that is simultaneously away from the pitcher's body and toward the target. When throwing a baseball, however, his pitchers apply a lot of upward force that results in extension that is away from the pitcher's body but toward the sky instead of the target.
[Added at 12:30 PM, Monday, April 27, 2009]
One of Dr. Marshall's pitchers from the video has emailed me and asked that I clarify the difference that I see. In short, Dr. Marshall writes that the driveline height of the baseball should be just above the ear. This description perfectly fits with the driveline height for the weighted exercises. In the high-speed video I've seen of Dr. Marshall's pitchers, the driveline height for their baseball pitches results in a release point that is almost a full arm's length above the ear.
Some exercise specialists believe that the extremely heavy weights train the muscles to fight gravity. As the pitchers step up the weight increments, their muscles learn more and more to oppose gravity. Holding up a 20-pound wrist weight requires more than 60 times the force that is required to hold up a 5-ounce baseball, so when one of Dr. Marshall's pitchers performs his arm action with a baseball, his body and arm are accustomed to applying much greater upward force than is necessary.
This is one explanation for the upward extension. It makes perfect sense even to a layman, but I don't believe it accounts for all of the differences.
Dr. Marshall himself focuses on the principle of specificity of training - that a pitcher should learn/train to pitch baseballs by pitching baseballs - so his interval-training programs make baseball pitching a part of the daily routine.
So why then do his pitchers have a distinctly different arm action when throwing baseballs compared to when they "throw" heavy weights? Truthfully, I don't have that answer. It could be related to old muscle memory, or I may have misinterpreted Dr. Marshall's ideal arm action.
If you know the answer, I'd love to read it.