A sinker is a very difficult pitch to perfect, and if that wasn't enough, yoiur coaching staff is going to expect a high percentage of effectiveness when and why it's called for.
That last part - when and why it's called for, can be a rough road for you, especially when you're looking for positive reinforcement of that pitch's effectiveness. In that regard, there are so many factors that influence your feedback, that in the amateur game you're lucky ... and I mean lucky, if you have at least 50% of those factors that prompt calling for that pitch.
- Your backstop being in the right distance from the batter to give you the proper perspective with respect to depth perception and location.
- Your pitching surface is of such quality that it promotes and reinforces your entire body to compliment your pitching cycle in its entirety, especially the follow-through part for a solid sinker.
- You're rested and prepared physically and mentality to command that pitch, in addition to being witness to the temperament of that pitch's location and movement ... or lack there of, and then making adjustments that work.
- Adjustments are your biggest concerns, given 90% of all that comes before that is in order. (* below I'll focus on one of the biggest*)
- There's a right time for a sinker, then there is a wrong time for a sinker. If someone else is calling the signs and they're not aware of the when and when not... you're in for a rough outing.
- Always remember that your bullpen session is your first inning of work. That is the time to determine what your effectiveness is of your pitch selection that you're bringing into the game that day/night. If you are still in the work-in-process phase of the sinker, be mindful of your selections - you must advise your coaching staff of what's working and to what degree.
Some pitchers are just natural sinker guys. In my book, Orel Hershiser was one of the best, a real artist with the sinker. On the other hand, if a batter can recognize the pitch patterns of a sinker ... he avoids lunging at the last second, thus taking away a lot of your effectiveness. Again, like I mentioned earlier, a sinker is either good/effective, or a sure bet hit served on a plate.
Now to address your question(s) more directly. Take a look below at the two seam fastball grip. It's your foundation for developing and "feeling" the formation of your sinker.
With a sinker, the grip is only a small portion of that pitch's attitude to the batter. Your follow through - during and after release is critical. This media is a poor substitute for showing you that, even video by someone other than yourself is marginal help at best. On the other hand, some say a picture is worth a thousand words... so, lets see if this can help you.. WHY a sinker won't work.
The statement that I made above, about your follow through is very important, and here's why:
Your follow through the release phase of your sinker pitch, MUST be somewhat the same design of your intentions when you first gripped the ball. See the picture above to the two seam fastball grip, well if you release your grip too early, without a fluid follow-through .... here's the way your release is going to look...
Notice how dramatically different this release grip looks compared to the picture illustrated in the two seam fastball picture. Your arm slot has a lot to do with this temperament also. A slingshot arm posture, from my experience anyway, is extremely difficult to manage, with respect to getting a sinker ... not impossible mind you, just very difficult. Again, this is my experiences only. So, that notation that I made above - adjustments, this is one adjustment that's very hard to recognize.
With respect to adjustments, sometimes ... and sparingly, a small adjustment to your grip may be the only manageable thing that you can muster at the time. Below is a pattern of movements of the fingers and thumb on the ball that you can practice with many pitches that involve some sort of movement. This exercise is called "dialing" the ball.
Now depending on a lot of other factors - too much to go into here, you may be able to work out a happy medium with your pitch, doing what you want it to do. Just be aware that his kind of fine tuning is not meant to correct problems with a bad form and the lack of preparation.
A good start to learning the fundamentals of this pitch - the sinker, can be found by watching this video by Jim Corsi.