How to throw a sinker like Roger Clemens
Throwing a good sinker will baffle hitters
The sinker, or sinkerball, is one of my favorite pitches. When thrown properly it'll look like it's changing direction soon after it leaves the pitchers hand. For a right handed batter facing a right handed sinkerball pitcher, it'll appear that the ball cuts down and inside. This can be very unnerving for a hitter, and it generally leads to a lot of weak groundball outs. Hard throwers can also use a good sinker to set up a fastball for a strike out. Roger Clemens was great at this. Current sinkerball pitchers include Fausto Carmona and Brandon Webb. Gaylord Perry also threw a nasty sinker that was likely aided by substances on the ball that would get him suspended if he were caught.
How to throw a sinker
The grip for a sinker is similar to the curveball grip, except your thumb and middle finger should be on the other side of the seams. For a right handed pitcher, that means you should have a seam parallel and to the left of your thumb and another seam parallel and to the left of your middle finger.
When you throw the sinker you need to pronate your wrist, which is fancy talk for turning it to the inside. If you imagine your thumb being underneath the ball and your middle and index on top of it, you're going to rotate your wrist and hand before releasing the ball so that your fingers end up on the left of the ball and your thumb down and somewhat to the right. It's basically a 90 degree turn of the wrist.
Why is the sinker such a great pitch?
Not all pitchers have Roger Clemens' power. The sinker is a great pitch because even pitchers who don't have great stuff can throw it and get a lot of guys out. Kent Tekulve pitched for the Pirates in the 1970's and 1980's. He didn't have great stuff, but he could get batters to hit into ground outs like no one else in the game. Despite pitching in over 1,000 games, he gave up less than 70 home runs in his entire career.