(Thanks, Foxsports, sorry for aggregating.)
It's the pitch they tell you not to throw. Growing up playing youth baseball the curveball is like a R rated movie, a cigarrete after school, or your friend's house with Cinemax. No one denies it sounds awesome, but it's verboten, off limits, taboo.
There is still debate as to what kind of dangers truly exist allowing developing bodies to torque their elbows, shoulders and wrists in such a way that the ball leaves the hand spinning in excess of 2600 RPM. What is not in debate, nor will ever be debated, is the thrill of successfully watching your friend completely miss your first truly good curve.
The motion of the catcher's glove is one I like to describe as "a lazy candy cane". The catcher knows which pitch is coming. Of course he does, he called for it. But even gifted with future knowledge the glove is still beholden to signals coming from the brain, which is getting its data from the eyes. Some curveballs have a graceful parabola. Others, like the one above, have a cliff-like plummet straight from the Six Flags Roller coaster from Hell. All, when thrown properly and married to the constant threat of a good fastball, are patently unfair.
"I became fully convinced that I had succeeded ... the batters were missing a lot of balls; I began to watch the flight of the ball through the air, and distinctly saw it curve."
While disputed baseball credits William "Candy" Cummings to be the first person to throw a curveball. Reportedly the inspiration came from flinging sea shells at the sea, which is a nice romantic image in a sport that loves romantic imagery. The quote above shows that even in 1867, when Cummings supposedly began first throwing the new pitch, a priority was placed on avoiding contact. Sabermeterics then are, perhaps, simply a return to common sense.
The curveball not only revolutionized the tools pitchers had when facing batters but the catcher position as well. In its infancy as a sport the catcher would stand far back behind the hitter. But Cummings' curve ball necessitated the catcher be located directly behind that hitter, the place where he has been now almost 150 years.
It only breaks about 3.4 inches, although our eyes tell us that break is more than 14 inches. Such is the way our perceptions marry reality. From it's initial conception and deployment pitchers have assembled their vast array of bedazzlements and witcheries. Sliders, splitters, slurves, change ups, fork balls, gyroballs, knuckeballs, and the eephus, among countless other variations, all owe their existence to the simple decision to reverse the baseball's rotation as it traveled plate-ward.
It's the curveball, a simple evolution of a simple game, nearly as old as our nation itself. It is evil, deceptive, beautiful, artistic, and potentially painful. It's just another small part of this, our baseball. And it is coming.