To hit the vertical center of the cue ball you must hit right on the red line which is a millimeter or so wide.
Aiming to hit either side of the line is easier because you have a larger target area to hit with the cue tip.
If you aim dead center on the vertical line and miss by a millimeter or two, on which side of the line are you hitting?
Sometimes you may hit to the left and sometimes you may hit to the right.
One side will be outside English and the other side will be inside English.
If I am aiming at the inside of the cue ball, I have the entire side of the cue ball to hit and if I stroke wrong and hit a bit to the wrong side I usually will hit center which will not impart the wrong spin to the cue ball.
It is either inside or center and my cue ball will always be spinning in the same inside direction or not spinning at all.
I don't have to worry about my cue ball going one way one time and another way the next time.
If I want it to go the opposite way, I will intentionally hit the other side of the ball...it won't be by accident.
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