The feet
A strong throw starts with the feet. The thrower should step directly toward the target (or maybe an inch or two to the glove hand side of a line from the thrower’s nose to the target). The step helps get the hips turning. If the step is to the throwing hand side then the hips cannot rotate to face the target. This reduces the strength of the throw. If the step is to the glove hand side by six inches or more, the body will rotate too soon before the arm is ready. This lack of coordination will result in a weaker throw and encourages wildness.
If the step is a short one, the shoulder cannot go very far forward. The throwing hand does not have very far to move during its acceleration of the ball and thus cannot put as much speed on the ball. A short step will often contribute to a high throw.
I have never known a player to have too long a stride on the throw.
The Hip Turn
The body rotation is generated by the legs. They are the strongest part of the body. If the hips do not turn, all the leg strength is doing is preventing the hips from turning against the throw. Therefore a strong hip turn with resultant rotation of the shoulders is necessary for a strong throw.
The hips should rotate as the throwing hand is coming forward since the hips are generating most of the energy for the throw and that energy must be present while the arm is moving forward.
The Shoulder Turn
The shoulder is like the handle of a whip. The arm and hand are the flexible part of the whip. The arc of motion of the whip handle determines the arc of the hand. Therefore, if the arc of motion of the shoulder is in a horizontal plane, the arc of motion of the hand must be nearly horizontal also.
If you stand directly behind a pitcher and concentrate on the throwing shoulder as a pitch is thrown, you can see that there is a lot of vertical motion of the shoulder when the ball is thrown well. The shoulder has an arc of motion that is at an angle of at least 30 degrees above the horizontal. To get that up and down motion of the shoulder, the back needs to be arched a little. An extreme arching of the back will throw the player off balance which will make the throw both weak and wild. The player should “throw out their chest” and “throw proudly” to get the proper arch of the back.
The faster the shoulder turns, the faster the ball will go.
The Arm
The elbow should be bent at least 5-10 degrees during the throw. If the arm is straight it is weak and liable to injury.
Changing the degree to which the arm is bent is like shifting gears. The further the hand is from the shoulder the higher the gear. The more the arm is bent the lower the gear. Since most players do not have a strong shoulder turn, the degree of elbow bend is unimportant if the arm is not straight or extremely bent.
The arm should continue it motion after the ball is released taking the hand below the knee. The full shoulder motion will have caused the thrower to bend at the waist allowing the arm to slow down gradually and bend at the elbow. This dissipates the energy in the arm without harm to the thrower.
If the hand is thrown at the target the arm will straighten with a “snap” and cause a mild hyper-extension of the elbow. This is what happens when the shoulder is stopped half way through the throwing motion. It is “cracking the whip” and is bad for the arm. You can see that this is happening when the thrower finishes upright and the hand does not go below the waist on the follow-through. The hand also rotates from a thumb to the side to a thumb down position which is a “twist of the wrist” that also hurts the arm.
The Hand
The hand should grip the ball on the finger tips (and the knuckle of the ring finger). The fingers on top of the ball should be close together though not necessarily touching. (It is OK if they touch.) The thumb and fingers should have a firm grip. The ball should not touch the palm of the hand at all.
As the throwing hand comes over the top on the throw the fingers should move through the center of the ball. This provides backspin to the ball. The backspin makes the ball resist falling in its flight through the air. This also provides maximum speed to the ball since the ball cannot be going slower than the finger tips. If the finger tips move down the side of the ball, they can go faster than the ball. This gives the ball a different spin.
As the hand comes toward the target, the palm of the hand will facing the target. If the hand is moving with a chopping motion the ball will tend to slip back between the thumb and index finger and go a lot slower than the finger tips.
The wrist
The wrist contributes so little to the throw and the speed of the ball that it should be ignored at this age. If you try to get a good “wrist snap” you will only make control of the throw more difficult. You will also get the occasional throw that hits the ground hard about half way to the target.
The wrist is too weak to make a noticeable contribution to the speed of the throw anyway. Try throwing a ball with your wrist firmly against a post and you will see that you can’t get any distance with it.