There was a question in the comments to "Picture, Imperfect?" that I realized was both common and important enough to do outside of comments:

FoulTerritory asked How does pronation help prevent elbow injury, or conversely, how does not pronating lead to elbow injury?

You'll have to go with me on this one. We'll use the curveball motion as the example. Get a hammer. Hit something a couple times with it, like a table or counter. (You don't have to hit it hard and certainly don't break anything.) You'll find that you "set yourself" to have the hammer hit the surface (and stop) just before your elbow "locks out".

Now, step away from the surface and swing the hammer in space, where you're not hitting anything. DO IT GENTLY and as if you're hitting something in front of you. Your arm will lock out and you'll probably feel tension, maybe even some pain. Your shoulder will lift slightly, the biceps will tense, the wrist will bend - all trying unconsciously to take the tension out of the elbow, which cannot extend further.

Final step - just as the elbow is about to lock out, pronate. Pronating means turn the thumb downward. If you're right-handed, the thumb will move anticlockwise. The elbow now has a path that goes from flexion to near-full extension, pronates, then extension to flexion again. The larger muscles of the arm and shoulder can now slow the hammer.

What does a hammer have to do with a curveball you ask? That's the same motion you'll do for either, including (roughly) hand position. So, short answer now demonstrated for you? Pronation keeps the elbow from locking out and causing damage.