I was wondering how some of these PRO Pitchers throw their elbow and forearm almost all the way back with keeping thier Shoulder, and Tricep at a 90 degree angle?
And does this addd extra velocity? And what can i do to practice that

I think you mean that point where the forearm is laid back to horizontal just as the shoulders have squared to the plate. This is what pitching circles tend to call “full external rotation”. If you stand with your upper arm horizontal (at shoulder height) and your forearm pointing straight up (the 90 deg. bend you mention) and try to pull your forearm back and down to horizontal with just muscular effort, you’ll notice that you can’t really get there. What gets pitchers there is a combination of the momentum of the elbow coming around (forward) and the inertia of the forearm, hand and ball. If you “push” the ball by getting the ball ahead of or even with the elbow, you won’t get to full external and you’ll experience a lack of velocity. It’s all a matter of timing. You need to look at this in a holistic manner. The energy built up by forward centre of gravity momentum (during the stride), elastic energy transfer (hip/shoulder separation building up tension in the torso), shoulder/elbow rotation and good arm action (whatever that is), all put together as a smooth kinetic chain with impeccable timing will result in this happening.

Sorry for not giving you a pat answer with a “do this” drill but I’m a firm believer in using a holistic approach to this stuff.

My opinion only, of course!!

This is a natural byproduct of throwing at high speed using the traditional pitching motion. You don’t need to practice it. It will just happen as you start to throw harder.

This happens when you start to turn your shoulders as your pitching forearm is vertical. The inertia of your forearm causes your shoulder to externally rotate (AKA “bounce” or “lay back”).

The problem with throwing this way is that it can subject your arm to tremendous forces as your elbow rapidly extends 90 or even 135 degrees. If you aren’t pronating while this rapid extension is taking place, then you run the risk of putting too much load on your UCL and slamming the bones of your elbow together.