The motion a pitcher goes through in the process of throwing a pitch is called a delivery. Each delivery begins with the pitcher being in one of two positions, which we will learn about in this tutorial.
The windup position is most often used when there are no runners on base. Since it takes longer than the set position, using the windup when there are runners on base would give more time for the runners to steal a base.
He keeps his pivot foot (his dominant foot and the one that will keep him grounded as he pitches) on the pitcher's rubber for the duration of the delivery. He then slides his free foot (his non-dominant/non-pivot foot) back, rotates his pivot foot to be parallel with the rubber, lifts his free foot up, and throws the pitch while striding toward home plate with his free foot. To finish the motion, the pitcher follows through by letting his arm continue to swing down after he has released the ball. His balance is now on his free foot, which has stepped toward home plate, and he kicks his back foot up as the momentum of the delivery drives his weight forward.
The set position, also called the stretch, is most often used when there are runners on base. It takes less time than the windup position, so it reduces the time a runner has to steal a base.
When a pitcher is in the set position, he begins with his pivot foot on the rubber, and with his feet and shoulders parallel to the rubber (left-handed pitchers face toward first base while right-handed pitchers face toward third base). With his dominant hand holding the ball and the other hand with the glove, the pitcher puts his hands together, signaling the start of the delivery. He then lifts his free (non-pivot) foot up, and throws the pitch while striding toward home plate. Just like with the windup position, the pitcher then follows through and kicks his pivot foot up.