According to a drill sheet regarding PSA by Storm, drilling into the MB enhances the dynamics of the weight block. What type of enhancements they mean is another question.
The reason the double thumb layout is so strong, is that by drilling the thumb and XH so close to the MB, you make that intermediate diff huge (comparatively). This is where the primary strength of the Double Thumb layout comes in. Thus, the diff ratio is at its maximum giving the reaction it does.
To maintain the diffs., the farthest away from the thumb hole that you can place the PSA is the center of the balance hole. To delay the breakpoint and still maintain the diffs., the center of the thumb hole is the farthest you can place the PSA in that direction.
