One of the niftiest tricks up the pitcher's sleeve is the ability to make a ball curve, or "break," in a variety of directions. When the ball spins at the proper speed around its vertical axis, the passing airstream exerts a deflecting force, caused by the rotation of the ball. Air travels with the spinning ball and is ejected on the opposite side of the spin (see diagram). Since we know that every action has an equal and opposite reaction (thanks to Mr. Newton), the ball is deflected in the direction of the spin, causing the ball to curve. And by controlling the direction of the spin, you control the direction of the curve.

The way a ball curves is determined by the direction and amount of spin placed on the ball, as well as the speed of the pitch. Fastballs usually curve slightly up, curveballs curve down and to the side, while screwballs, with a spin opposite to that of curveballs, break the other way.

The everyday "pretzel-bender" curve is thrown with a good snapping spin down and to the right (for a right-handed thrower). The axis of spin is tilted, so the ball curves down and away from a right-handed batter. Since the ball breaks on two different planes at once, it's much harder for a batter to judge its trajectory. The screwball is a curve ball that breaks in the opposite direction. The wrist and fingers are snapped in toward the pitcher's body, so the ball curves in and down to a right-handed hitter.

The slider, or "nickel curve," is thrown harder than a normal curve, with the wrist cocked at a 90-degree angle -- not unlike a spiral football pass. The movement is slight, but the effect is devastating. The batter's difficulty in adjusting for the small curve at the end of the ball's flight makes for weak pop-ups and ground balls -- easy outs. When the slider first appeared in the 1950s, batting averages plummeted. Corrective action was taken by lowering the pitcher's mound and creating a smaller strike zone, but this has not totally neutralized the slider's nastiness.

Just when and where a ball starts its curve is affected by the speed at which it is thrown. At low speeds, the flow of air over the front surface of the ball is smooth, but turbulence develops toward the back, where the smooth flow breaks away. At high speeds, on the other hand, the layer of turbulence is distributed evenly all over the front surface of the ball. But as the ball flies through the air, it slows down; at a critical moment, the turbulence in front is suddenly replaced by a smooth flow. If the ball is spinning, this critical moment will occur an instant sooner on the side moving away from the direction of flight. Since the ball's propensity to curve depends upon a layer of surface air being carried around the ball, this process is initiated on the side with more smooth air and delayed on the side with more turbulence. The onset of the curve is thus quite sudden.

How well can pitchers control the critical speed factor? Some control it very well indeed -- the late, slight "break" of the slider is what makes that pitch so effective, and speed is the key. What looks like a fastball actually slows down enough to start curving just inches in front of the plate.