At a practice I noted kids throwing a light batting machine ball. Most of the kids could throw it really hard and my question is this-

1.Is underweight throwing good or bad on the arm?

It seems like to me that the kids who could fluidly throw the ball in a rapid motion are the hardest throwing baseball pitchers ona team.

2.Is underweight throwing a good analytical tool for detecting throwing flaws that otherwise are hard to pick up?

Underweighted and overweighted balls have been used for strength and conditioning for the arm for a while. Not used to my knowledge for throwing analysis purposes. In fact if someone has flaws in their throwing they can probably do more harm than good. Probably not a good idea for younger kids in particular.

Underweighted balls are usually limited ot 4oz due to the concern of throwing extremely light balls (like wiffles/tennis/etc) can be hard on the arm.

I would chalk up your observation to the Hmmmmm.... catagory and teach the fundamentals of proper throwing.

I read somewhere that mimicking the throwing motion with no ball will increase arm speed and that encourages fast twitch muscles. You then increase the weight in your hand and gradually your arm will move at the no load speed. I guess there is some merit to it. All I know is.... throwing more and long toss make your arm stronger and that will keep your arm healthy.

Now what the heck are "fast twitch muscles"?

The way I understand it is the body has 2 kinds of muscle mass, fast twitch and slow twitch. Fast twitch are lean and long and generate speed --like a track star vs. a body builder. The theory is if you work out to bulk up you will lose your fast twitch muscles and slow your body down. Baseball is a game of speed...arm speed,bat speed and foot speed. So you want to promote fast twitch muscles by not training at half speed or overload. Fatiguing the muscles create bulk so they say to do fewer reps at maximum speed.