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From BR Bullpen
A screwball is essentially a reverse curveball. While a curveball, unless thrown straight overhand, tends to break away from a same handed batter, a screwball tends to break toward a same handed batter and away from an opposite handed batter. This reverse motion is especially useful for left handed pitchers, so it's unsurprising that a disproportionate share of screwball pitchers were left-handed.
While the screwball is described as a reverse curve, the effort and effect on the pitcher's arm are very different from a curve. A curveball involves rotating the forearm so that the pitcher's palm is facing his body, the screwball requires turning it the other way so that the palm is facing outward. This motion is inherently very stressful, a point that has often been mentioned by its practitioners:
- Christy Mathewson, whose fadeaway pitch was probably the first well-known screwball, commented that he could only throw it a few times a game because it took so much out of his arm.
- Carl Hubbell's throwing arm was permanently deformed by throwing the screwball.
- Jack Coombs, in his classic instructional book Baseball: Individual Play and Team Strategy, cautioned against teaching young pitchers the screwball because "it calls for muscle movements in the wrist, forearm, and elbow that are contrary to the laws of nature."
The screwball is rarely thrown today. Recent practitioners include Mike Marshall,Tug McGraw, Enrique Romo and Fernando Valenzuela. The key reason for its demise is probably the development of the circle change, which has similar movement but is much less stressful to throw.
A disproportionate number of screwball pitchers have been left-handers such as Hubbell, Valenzuela and Masahiro Yamamoto. As of 2007, the last known screwball pitcher in Major League Baseball was Jim Mecir of the Florida Marlins.
 Further Reading
- Warren Corbett: "Hubbell's Elbow: Don't Blame the Screwball", The Baseball Research Journal, SABR, Volume 40, Number 2 (Fall 2011), pp. 23-26.