A cutter is a version of the fastball, designed to move slightly toward the pitcher's glove side as it reaches home plate -- a cutter from a right-handed pitcher will "cut" away from a right-handed hitter, or in toward a left-handed hitter. Cutters are not thrown by as many Major League pitchers as four-seam fastballs or sinkers, but for some of the pitchers who possess a cutter, it is one of their primary pitches.
Although cutters are a form of fastball, a pitcher will often throw a cutter in addition to his four-seam fastball or sinker, with the cutter having a slightly lower velocity. For some pitchers, their cutter is more like a breaking ball -- almost a hard slider -- than a fastball.
A pitcher with an effective cutter can break many bats. When thrown from a right-handed pitcher to a left-handed hitter, or a lefty pitcher to a righty hitter, a cutter will quickly move in toward a hitter's hands. If the hitter swings, he often hits the ball on the smaller part -- or handle -- of the bat, causing it to break.
In rare cases, switch-hitters have been known to bat from the same side as the pitcher's throwing arm when that pitcher throws primarily cutters. (Typically, a switch-hitter will hit from the opposite side of a pitcher's throwing arm.) This is because the unique movement on the cutter causes hitters to get jammed when facing a pitcher of the opposite handedness.
A cutter is thrown like a fastball, but with both fingers placed together toward one side of the baseball (the right side of the ball for a right-handed pitcher, or the left side for a lefty) to produce the cutting movement once the pitch is released.
The key to a cutter is deception. Batters are accustomed to facing either straight four-seam fastballs or two-seam fastballs that break toward the pitcher's arm side. The cutter breaks in the opposite direction of a two-seamer, and it does so very late in its journey to home plate. This movement is designed to make sure the hitter isn't able to hit the pitch squarely.
The cut fastball has been thrown for more than 50 years, but it was made famous by Hall of Fame Yankees closer Mariano Rivera, who threw the pitch almost exclusively. Rivera's cutter had so much late movement, it gained fame for the sheer number of left-handed hitters' bats that it broke.
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