Throw it with the 2 fingers on the seams.
Then place the middle finger back so the pointer is farther on the ball and the fingers are even or the pointer is more ahead.
Put the thumb on the side of the ball
Grip the ball firm not loose
RELEASE: Release the ball down and In.
PS make sure you throw off the middle finger last.
Hudson's ball sinks because of his release, not just because of the grip.
I think what's important is not that the fingers are on the seams but that the fingertops are NOT on the seams.
But I also agree with palo20 on the importance of the release.
what do you mean when you say finger tops not on the seam?
you want the tips of your middle and index fingeres on the leatehr of the ball, not the seams.
so you don't want the whole finger on the ball?
Only the tips and then a gap because you only put the tips on?
No, the whole finger is on the ball (although you're certainly free to try new grips). It's not about what is or isn't touching the ball - it's about positioning on the ball. The finger tips are simply positioned on smooth leather - not on the seams.
Backspin has a lifting effect. Velocity gives gravity less time to push the ball down. The idea behind the sinker it to reduce both backspin and velocity a bit (though not at much as a change-up). Not using the seams reduces spin and lets you take something off the pitch.