In baseball, sidearm describes balls thrown along a low, approximately horizontal axis rather than a high, mostly vertical axis (overhand).
Sidearm is a common way of throwing the ball in the infield, because many throws must be made hurriedly from the glove after fielding ground balls. An infielder’s quickest throw to the bases is often from just above ground level, necessitating a horizontal release of the ball.
Sidearm pitchers, also known as sidewinders, are uncommon at all levels of baseball (except in Japan, where sidearm pitchers are widely popular). Few find sidearm a natural delivery, and those who do are often discouraged by coaches who know little about sidearm mechanics, and who believe that overhand pitching affords greater velocity. This is generally true, since a high release point uses gravity to accelerate the ball, even as air resistance works to slow it. With a low sidearm release the ball is slowed threefold: by gravity (as it ascends), increased distance (because of its higher arc), and air resistance. But what the sidearm pitcher loses in velocity, he gains in ball movement and unusual release point.
Read more about Sidearm: Historical Progression of Pitching, Ball Movement, Release Point, See Also