There are a couple of things going on.
The fastest movement a body can make, other than blinking, is the internal rotation of the dominant hand.
Try it...If you a rightie, extend your right hand in front of you with your thumb pointing to the right. Then rotate the hand quickly so that the thumb is pointing to the left. Pretty fast, isn't it?
There is a game you used to play where you put your hand under another kid's hand. You flip your hand over and slap the top of the other player's hand. The other player tries to pull his hand away. The rotation of the hand is quicker than pulling the hand. (If you want to have your hand beaten to a pulp, play this game with a D1 pitcher. You don't have a prayer.)
Next, get open, put a ball in your hand, and put your arm behind you at 9. Put the ball facing down. Now, flip the hand so the ball is up. Feel the biceps stretch when your hand is under the ball as opposed to the hand being on top of the ball?
Essentially, the bicep becomes a rubber band for the arm. To propel rapid internal rotation of the arm.
The whole thing about "ball up at 9" is to stretch the bicep so that during the release phase the forearm rotates as fast as possible. If the timing is right, the ball is thrown with greater velocity.
This is good info on IR Sluggers. Thank you!