Most pitchers throw a bad sinker because they try to rotate too early — and that one mistake kills all the movement.
When I was learning the sinker, my whole cue was to rotate my right shoulder as far as possible BEFORE I deployed my arm.
Like, I thought my arm was just going to hang there and do nothing while I kept rotating.
That sounds weird. But it works.
If you rotate and throw at the same time, the ball goes the wrong way. You lose the backdoor shape, you lose the sink, you lose everything.
The sequence is: foot down, rotate, rotate, keep rotating — THEN deploy the arm.
That last little bit is just getting your hand out front.
The best place to train this is catch play. More reps, lower stakes, faster feedback.
You can start with the grip already set and just work on that final rotation piece. Focus on reaching forward THROUGH the rotation — not reaching sideways, not muscling it — just letting the hand come through at the end of a full shoulder turn.
Once you feel what it's like to rotate first and then throw out front, it's easy to bring that into your full delivery.
The feel is the whole thing. Get the feel in catch play. Then scale it up.