The midline is a line through the center of grip perpendicular to the finger/thumb line. It divides the ball into two halves, the finger half and the thumb half. When people talk about finger weight or thumb weight, they mean that one of those halves is heavier than the other (by up to 1oz).
Putting the CG on the midline, whether it's close to the grip or swung out, balances the finger and thumb weight naturally. Putting the CG high above the midline gives finger weight, below thumb weight. If you have too much finger or thumb weight, you need to add a weight hole above or below the midline (respoectively) to remove it. You can also drill the fingers or thumb deeper to remove excess finger or thumb weight.
The easiest thing to do, though, is put the CG close to the midline. It's why long pins tend to get placed above the fingers and why short pins tend to get placed below them.
SH
