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The Splitter (aka Split-Fingered-Fastball) is a pitch thrown with the same motion as a fastball, but with the index and middle fingers split wide, each contacting the ball on opposite sides. The grip is similar to the forkball except that the ball is not held as deep into the hand as it would be on a forkball.
The splitter has a slow, forward tumbling rotation as opposed to the fastball's backspin. Because the middle finger is usually slightly longer than the index, the forward rotation ususally favors the middle finger side. That means that instead of rolling over the top straight towards the middle of home plate, it will roll forward and toward a right-hand batter if thrown by a right-hand pitcher. This uneven rotation causes the ball to break down and to the side of the pitcher's hand.
The forkball is held deeper in the hand and can't be thrown as hard as a splitter. If a pitcher's fastball is 90 mph, his traditional forkball will be at 82 mph or so while his splitter will be 86-88 mph. This makes the impact of a splitter is less a change of pace, as a forkball would be, and more a fastball with unexpected movement.
Some split-finger grips include the thumb contacting the underside of the ball. This serves to prevent backspin, which is necessary to get the proper break, but may or may not be necessary depending on hand and finger size. Contrary to popular belief, the splitter puts no undue strain on the shoulder or elbow. It can be learned relatively easily and controlled fairly quickly. The spitball or grease-ball, which has movement similar to the splitter, can not be learned or controlled as easily, which is why the spitball is rarely used today.