Here's how to throw a sinker.
Place your thumb between the seams on the smooth leather on the opposite side of the baseball (see pic at right).
Keep your hand and fingers on top of the ball until it's released. If a pitcher "cuts" the ball or releases it so that the hand ends up on the side of the ball, the sinker doesn't do much sinking.
Friction is created when the ball is thrown. Friction is what makes the ball sink. It's also what makes the pitch a few miles per hour slower than a normal fastball.
A good sinker is thrown down in the zone. Even if it's in the middle of the plate, if it's sinking down, you'll get a lot of ground-ball outs.
The sinker is an ideal pitch for younger pitchers. Its simplicity makes it an easy pitch to learn, and the way in which it is thrown puts little stress on young arms.
There's not much twisting of the wrist, so there's little torque put on the elbow or shoulder.
The pitch doesn't require much velocity to move down in the strike zone, so young pitchers won't need to "muscle up" on the ball and potentially break down in their pitching mechanics.
Sinker grip (side view)
Most of the time, you'll want to throw a sinker with your fingers parallel with the seams. Sometimes, depending on finger pressure, it could be on one seam or it could be on the other seam.
A lot of guys throw a sinker off of the middle finger. The movement is down and late. It sinks, for a righthanded pitcher, into a righthander, and away from a lefthander. For a lefthander, it's into a lefthander, away from a righthander.
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