Well, in the "old" days, topweight was something we used to promote length and backend and bottomweight was used to create earlier roll and smoother backends reaction.
This was all before very dynamic weightblocks, pin placements, mass bias' and pap locations came into relevance. These days, it is thought that static weights have no, or minimal, effect on the rolling characteristics of a ball.
I, myself, think this may be true of finger/thumb or side weights, but also think that top, or bottom, weight may still have some influence.
I really don't know the specifics of WHY this happens, only practical experience that it does for me.
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ILLEGITIMI, NON-CARBORUNDUM!