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 (hopefully) 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" 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, and that spin creates a high pressure air zone ahead 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 diminished 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.
Conversely a curveball, thrown with downspin, will create that high pressure zone on top of the ball, and push the ball down in flight. Combined with gravity, this will give 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 in its axis of spin.
At the professional level, a curveball is often 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 curves movement, serves 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’s 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 balls raised seams in 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”.
A popular nickname for a curveball is the "deuce," referencing the fact this is commonly the No. 2 pitch in a pitcher's repertoire. A catcher will often use a two finger signal when requesting the curve. Other popular nicknames for the curveball include: "hammer", "bender", "hook", "yakker", "Lord Charles" and "Uncle Charlie".
Baseball lore has it that the curveball was invented in the late 19th Century by either Candy Cummings 1 or Fred Goldsmith. An early demonstration of the "skewball" or curveball occurred at the Capitoline Grounds in Brooklyn in August of 1870 by Fred Goldsmith. In 1884, St. Nicholas, a children's magazine, featured a story entitled, "How Science Won the Game." It told of how a boy pitcher mastered the curve ball to defeat the opposing batters. In the early years of the sport, use of the curveball was thought to be dishonest and was outlawed, but officials could not do much to stop pitchers from using it. In the past, major league pitchers Bob Feller, Virgil Trucks, Herb Score, Camilo Pascual and Sandy Koufax were regarded as having outstanding curveballs. Bert Blyleven is said to have had the best curveball in the recent modern era . Other notable pitchers who throw or threw great curveballs since 1900 are/were, Sal Maglie, Dwight Gooden, Nolan Ryan, and Barry Zito.