In baseball, the sinker is an effective pitch for inducing ground balls from the batter. Because the sinker is similar to a fastball but with a drop at the last second, batters tend to swing over the pitch, thereby causing the ball to hit the ground immediately off the bat. It is not difficult to throw a sinker, but keep in mind that this pitch can cause stress on certain arm muscles and is not recommended for younger pitchers.
To throw a sinker, hold the ball in a similar fashion as a fastball, with your index and middle finger parallel to the seams. Your follow-through when you throw this pitch should be overhand and similar to a fastball motion, but many pitchers will put extra pressure on their middle finger, which keeps your hand in contact with the ball just a bit longer. This will cause extra spin that will make the pitch drop as it crosses the plate. Typically there is incidental horizontal movement when you throw a sinker as well.
Pitchers will also send their wrist downward when they throw a sinker in order to cause a sharper sinking motion. While effective, dropping the wrist downward also increases the risk of wild pitches. In addition, this motion can cause undue stress on the wrist, thereby increasing the risk of injury. One other drawback to pitchers who throw that pitch occurs when the pitch is not thrown correctly and therefore becomes a fastball. Most hitters will react to such a pitch and, if able to get a good portion of the bat on the ball, can drive the pitch into the outfield or beyond.
Because the pitcher's motion is similar to that of a fastball, batters are often fooled by the pitch. Pitchers who throw sinkers do not need to change pitch speed as much as other pitchers because the sinker is so effective. Other pitchers might use off-speed pitches like a breaking ball or change-up to fool hitters, but pitchers who throw a sinker well can rely on the pitch heavily and often induce harmless ground balls.