The direct correlation between a sound, repeatable pitching motion and success on the mound can't be overstated. As difficult as pitching can be, the many variables around the game can make it that much harder (how good is the hitter, is it raining, is there a huge hole on the mound). The pitcher needs to learn what they can and what they can't control. This starts with their delivery.
BEFORE THE DELIVERY
An often overlooked fact amongst youth pitchers is that every throw counts toward the overall makeup of your pitching delivery. The principles of muscle memory, which are often applied to hitting in the cage or working off a tee, affects your game and tend to go unnoticed in the scope of developing sound pitching mechanics. Pitchers need to be mindful of what their arms and bodies are doing. This isn't something just applied while pitching in the game or working in the bullpen, but every catch session and long toss all works in unison with your muscle memory as a pitcher. Play catch with a purpose, not just to get loose.
WINDUP STANCE
Before even stepping on the rubber, the pitcher needs to know which stance is appropriate for their current situation. With no runners on base, for example, the windup can be used. For this stance, the pitcher will begin on the rubber facing home plate, with their feet roughly shoulder width apart and their glove and throwing hand together in front of their chest. The pitcher should be upright and relaxed. From this position, the pitcher will take a short step back with their glove-side foot (this small step back does not represent a transfer of weight and should simply be used as a rocker step). The pitcher will then pivot their throwing-side foot so that it is flush against the front side of the rubber. Once in this position, the pitcher can then pull their glove-side knee up to get to the leg lift or balance position.
STRETCH STANCE
The stretch stance is being used more commonly today than ever before amongst pitchers with no runners on base. This stance must be used with runners on, and its simplicity lends it to frequently being used to introduce pitching to younger players. For this stance, the pitcher will start with their glove-side shoulder pointed toward home plate, and the outside of their throwing-side foot flush with the front side of the rubber. Their glove and throwing hand should be in a relaxed position in front of their chest, and their feet should be shoulder width apart. From this position, the pitcher can shift their weight back onto their throwing-side foot to get to the leg lift or balance position.
LEG LIFT OR BALANCE POSITION
No matter which stance is used to start the delivery, the pitcher will always gain their balance and collect their power using a leg lift, which is also known as a balance position. The pitcher will bring their glove-side knee up so that their thigh is at least parallel to the ground and they are balanced on their throwing-side foot. Their head and eyes should be aligned, and their shoulders should be parallel to the ground. While maintaining balance and body control, their lower body should be loaded and ready to move forward to deliver the pitch.
THE BREAK
The pitcher begins their movement toward the plate with the break phase. In the break phase, the pitcher should be focused on the catcher's mitt. This is when the ball and glove separate, and the throwing motion begins. The glove, throwing arm, and stride should all move in synchronization with each other. It is important that the pitcher's head stays over their throwing-side foot when they begin their movement forward. The glove-side elbow should be used to pull the throwing shoulder through. The throwing hand fingers should be on top of the ball, with the arm working through a down, back and up progression so that the ball is ultimately pointed toward the typical shortstop position for a right-handed pitcher, and the typical second baseman position for a left-handed pitcher. The stride should be with the glove-side foot and made in line with the target. The stride foot should land even with the throwing side foot that is engaged with the rubber.
POWER POSITION
The glove-side foot has now hit the ground, leaving the pitcher in the power position. Here, the legs are in a wide base, both arms are raised and prepared to begin the transition forward. This transition should begin as soon as the glove-side foot hits the ground. The glove-side hand will begin pulling into the pitchers glove-side chest, while the throwing arm continues the arc, now forward toward home plate. The throwing-side elbow should be at least at shoulder height, with the throwing hand behind the ball. The motion of the glove being pulled in and the throwing hand moving forward happen simultaneously to maximize both velocity and accuracy. During this time, the pitcher's upper body also shifts from its starting point over the back leg toward a more advanced point over the front leg. This is a quick acceleration and shift in momentum culminates in the pitcher reaching his release point.
RELEASE POINT
The release point is where the pitcher's entire delivery comes together. Now, his upper body and momentum have shifted out over his glove-side leg. The throwing-side arm is now working out in front of his body and is accelerating toward the catcher. The ball should be released out front on a downward trajectory toward the target. The glove hand is firmly tucked into the pitcher's chest and his momentum will carry forward in to his follow through.
FOLLOW THROUGH
Having now thrown the pitch, the proper execution of the follow through is a critical last step to the delivery. There are two primary tenants to a proper follow through. First the follow through should enable the pitcher to have a natural deceleration of his throwing arm. An abrupt stoppage of the arm or an off-balance finish is a recipe for an injury. The pitcher wants to finish his motion with a flat back and allow his throwing arm to finish past his glove-side knee. Secondly, a good follow through will land the pitcher in a good position to become an active fielder. Fielding his position is one of the best ways a pitcher can help his own cause and the proper follow through is the first step to achieving that success.