Every major league pitcher has an out-pitch -- the weapon of choice when it's time to put a hitter away. It's just that some are better -- sometimes, much better -- than others. That's what separates the great pitchers from the merely average.
An ordinary 88-mph fastball isn't going to do the job -- unless it features extraordinary movement. Often, an out-pitch is a breaking pitch or off-speed pitch, which makes recognition tougher while disrupting timing.