A sinker is a pitch in baseball which will drop right on the back of the plate. It will take practise to perfect the shot but it is worth the effort.
The first thing to concentrate on is to hold the ball correctly. You need to run your index and middle fingers along the seams and then lay your thumb under the ball lightly just to keep it balanced in your hand. When you throw the ball do not bend the wrist and keep the hand on top of the ball until you actually let go of it. The lower you throw it the more it will drop and the faster the speed of your arm the more break. If you flick your wrist it will help with the drop.
As long as you can keep the ball down in the strike zone it will help to keep the ball down on the ground. If you throw the ball too high it will not sink. When you throw the ball make sure you roll it off the middle and index fingers.
If you have already had two strikes then this is a great shot to try as it is likely to distract the batter and enable you to get him out. It is a good idea to throw it low and inside aiming at the batters waist.
This shot is a more unusual one and may not be one that you have practised much. This means that you will have to put in a lot of work to be able to get it right. However, as it is unusual then the batters are unlikely to be expecting a shot like this and so it is worth perfecting as it can make the difference in winning and losing a match. A few good shots can turn the match right around.