The sinker is a pitch with hard downward movement, known for inducing ground balls. It's generally one of the faster pitches thrown and, when effective, induces some of the weakest contact off the bats of opposing hitters.
Sinkerballers -- pitchers who rely on the sinker -- are adept at inducing ground balls and limiting home runs. Such is often possible due to the sinker's sharp downward movement, which is conducive for inducing weak contact.
Watch: John Smoltz demonstrates how to throw a two-seam sinker.
According to noted sabermetrician and writer Bill James, while the sinker existed before the 1950s, pitchers didn't explicitly try to throw it. They simply threw their fastballs -- and a select few of them had a sharp sinking movement on them. It is widely believed that around 1950, pitchers began to intentionally incorporate a wide range of movement on their fastballs. That's when the sinker finally came into its own.
In A Call
"sinkerball," "ground-ball pitch," "sinking fastball"