A cutter is a version of the fastball, designed to move slightly away from the pitcher's arm-side as it reaches home plate. Cutters are not thrown by a large portion of Major League pitchers, but for some of the pitchers who possess a cutter, it is one of their primary pitches.
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.
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 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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