The tricky thing about a fastball is that at the major league level it needs to be more than just fast. It has to move too, and be thrown to pinpoint locations. Otherwise, it becomes the hitter's advantage. Throw the ball too straight, and top professional hitters will eventually dial in on it -- no matter how fast it's coming at them -- and send the ball for a long ride.
Pitchers get movement on their fastball by using different grips. We just saw the four-seam fastball, and here we have the sinker, a variant of the two-seam fastball. This pitch sinks on its journey toward the plate. Because it sinks, hitters will tend to catch the ball off the top half, hitting it toward the ground instead of the air.
When good sinkerball pitchers throw, the infielders behind them will really get a workout. Pitchers like Atlanta's Tim Hudson -- to whom the pitch has been very, very good -- get lots of ground-ball outs by throwing sinkers.