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Today most pitchers don’t throw the screwball because of the widespread belief that they are going to get hurt.

NYT: Hector Santiago of the Los Angeles Angels was sitting at a restaurant table in Glendale, Ariz., in March, holding an orange in his left hand. He formed a circle with his thumb and forefinger, then spread his remaining fingers around the fruit with half an inch between each one. He was demonstrating how he throws his screwball, which is the best in baseball mostly because nobody else has one.

The secret, he said, is to exert no pressure with the pinkie or ring finger. As he moved his arm forward in a slow-motion simulation, he pushed hard with his middle finger on the inside of the orange until much of his hand was beneath it, creating a clockwise spin. “Like driving on your right wheels going around a curve,” he said.

Earlier that day, in a spring-training game, Santiago, a 26-year-old southpaw from Newark in his fourth season, threw a screwball to the All-Star outfielder Carlos Gomez of the Milwaukee Brewers. His previous pitch, a fastball, hit 94 m.p.h. The screwball approached the plate at 76. The difference in velocity alone would be difficult for a hitter to process, but the clockwise spin on the screwball also caused it to drop precipitously and veer to the left, away from the right-handed Gomez rather than toward him, as a curveball would. Gomez swung mightily and missed. “That pitch was filthy,” he told me later. “I was looking for it. I had it. And it disappeared. Put that guy on ice, man. He’s going to win a lot of games."

http://www.nytimes.com/2014/07/13/magazine/the-mystery-of-the-vanishing-screwball.html