Ball Movement
The various spins pitchers commonly employ—fastballs, curveballs, sliders, cutters—cause the ball to diverge from a “normal” trajectory. This is caused by the Magnus effect, which makes the ball move in the direction of its rotation. Batters learn these spins and their likely trajectories, but predominantly from high-axis pitchers whose pitches rotate around a mostly horizontal axis. Sidearm pitches rotate similarly, but around an approximately vertical axis. This causes common pitches to behave very uncommonly. For example, the Four Seam Fastball, when thrown by overhand power pitchers, seems to “hop”, or rise on its way to the plate. This is because the ball is rotating backwards, lowering the air pressure above the ball. The same pitch thrown by the sidearm pitcher causes a horizontal rotation, and consequent sideways movement. And sidearm pitchers whose deliveries are below the horizontal (see Submarine (baseball)) throw a fastball that rotates nearly forward, so the ball will sink rather than rise.
Read more about this topic: Sidearm
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