In the previous post, Jonathan Scippa commented on Ross Ohlendorf's excellent slider. The changeup is the evil twin of Ross's slider. While batters hit just .131 and slugged .237 off the slider, they creamed the change for a .352 BA and a .730 slugging percentage (all stats in this post cover 2008-2010).
The changeup is supposed to look like a fast ball. To fool the batter, the pitcher uses the same arm speed and release point as the fastball, but holds the ball differently so the speed is slower. The batter swings at what he thinks is a high speed pitch, but the ball isn't there yet. A poor changeup, however, gives the batter an easy pitch to hit.
As you might expect, the slower pitch drops more than a fast one:
Ross's fastball, on average, moves just as you would expect a pitch to move. You might call it flat. Hitters, in fact, do well against his fast ball, hitting it for a .306 BA and a .495 slugging percentage.
Note that the changeup is anything but flat. It's moving down at the plate, just what a batter wants to hit a home run. Ross's fastball get hit 315 feet on the fly; his changeups go 323 feet. Ross gets the double whammy with the change. He doesn't throw it well enough to fool batters into thinking it's a fastball, and the drop makes it a great pitch to hit out of the park. He should stick with the slider.