To throw a sinker in baseball, place your middle and index finger between the seams of the ball at their closest point, and lightly tuck your thumb under the ball. Keep your fingers on top of the ball until you release it, and try to put topspin on the ball. Follow through after releasing the ball, allowing your pitching thumb to almost graze your lead leg.
To be effective, aim a sinker on the low side of the strike zone. The lower the ball is thrown, the more it will drop. Throwing the ball faster or bending your elbow also causes it to break more.
An alternate way to grip the ball for a sinker is to place your middle finger between the two narrow seams and your index finger slightly outside the seam. Gently rest your thumb on the bottom seam. Throw the ball, and as you release it, apply pressure mainly with your index finger.
The ideal sinker appears to the hitter to be a fastball and then suddenly drops 3 to 6 inches. A batter typically hits a ground ball when making contact with a sinker. Some pitchers throw the occasional sinker to throw off a batter, while others who excel at the pitch rely on it heavily.