A fastball typically has backspin, giving it relatively stable aerodynamic characteristics in flight. The spin of a curveball moves in the opposite direction. This spin causes a curveball to "break", or drop down and sweep horizontally as it approaches home plate, thus frustrating the batter.
When throwing a curve, the pitcher creates downspin by rolling his palm and fingers over the top of the ball while releasing it. The direction of the break depends on the axis of spin on the ball. There are many variations of the curveball, but most are described in terms of their movement when superimposed on a clock. A "12–6" or "overhand" curve has a more or less straight downward action as it approaches the plate, while more sweeping curveballs might be described as "1–7" or "slurves". There is no specific point where a ball breaks, but the deviation from a fastball trajectory becomes progressively greater as the ball approaches the plate.
Generally the Magnus effect describes the laws of physics that make a curveball curve. A fastball travels through the air with backspin, which creates a high-pressure zone in the air ahead of and under the baseball. The baseball's raised seams augment the ball's ability to churn the air and create high pressure zones. The effect of gravity is temporarily counteracted as the ball rides on and into energized air. Thus the travel of a fastball is more or less straight, at least over the distance from the mound to home plate.
On the other hand, a curveball, thrown with topspin, creates a high-pressure zone on top of the ball, which deflects the ball downward in flight. Combined with gravity, this gives the ball an exaggerated drop in flight that is difficult for the hitter to track. The curveball may have some horizontal movement as well, depending on the tilt of its axis of spin.
At the professional level, a curveball is usually about 15 miles per hour slower than a fastball. Curveball behavior is unique to each pitcher though, and varies. Some use a more looping slow curve and some use a harder, faster slurve. The speed difference between a curveball and fastball, as well as the curveball's movement, serve to deceive the batter. Ideally, a curveball will have its greatest break just as it reaches the plate and cause the batter to swing above it.
To throw a curveball correctly, proper spin must be given to the ball as it is released. Generally pitchers grip the ball deeper into their palm and fingers than they would a fastball. Pitchers usually position their index finger aside one the ball's raised seams for more leverage in spinning the baseball. At the release point they then roll their hand over the top of the ball to throw it forward with downspin. If this movement is poorly executed the ball will have lazy spin, not break in flight, and be much easier to hit—the "hanging curve".
When thrown correctly, it could have a huge break from seven to as much as 20 inches.
A popular nickname for a curveball is the "deuce", since it is commonly the number 2 pitch in a pitcher's repertoire. Catchers often use a two-finger signal when requesting the curveball. Other popular nicknames for the pitch include: "hammer", "bender", "hook", "yakker", "Public Enemy No. 1", and "Uncle Charlie".
A curveball, because of the risk of injury to the pitcher’s elbow and shoulder, could be considered a more advanced pitch; geared more towards pitchers with more developed and mature arms, the curve ball can be very effective, yet very dangerous. It is suggested that the pitcher be close to 15 years old before attempting a curve ball. It is not a matter of being able to learn how to throw it, but it is a matter of the maturity of the pitcher’s arm. Important factors to consider before learning how to throw a curve: Has the child hit puberty; How developed are the child’s muscles and connective tissues, i.e.—ligaments, tendons, and bones; Is the pitcher in the middle of a growth spurt; Has the pitcher been taught the correct mechanics in order to throw a curveball? The parts of the arm that are most commonly injured by the curve ball are the ligaments in the elbow, the biceps and the forearm muscles.
In 1949 Ralph B. Lightfoot, an aeronautical engineer at Sikorsky Aircraft, used wind tunnel tests to prove that a curve ball actually curves and is not an optical illusion. Lightfoot is in the Baseball Hall of Fame for this work.
Regardless of the evidence, some viewers over the years remained convinced that the curveball was an optical illusion. Baseball Hall of Fame pitcher Dizzy Dean has been quoted in a number of variations on this basic premise: "Stand behind a tree 60 feet away, and I'll whomp you with an optical illusion!"