Got this info off
http://bowlingball-info.com/Ten-pin-bowling-technology.php
Not sure exactly what year and if this is correct info on what your looking for. It's the best I could find.
At some point in ball making and drilling the USBC introduced ball balance regulations to prevent people from taking advantage. It was possible to drill the grip at a location relative to the weight block so that it would achieve some effect, such as to help the bowler make it roll earlier or hook more.
Prior to about 1990, the USBC "static" ball balance regulations were adequate. The core was usually a uniform sphere centered inside the ball. Then competition among ball manufacturers motivated the production of balls designed to offer more than the "static balance" tricks. Materials and fabrication changes have since allowed the assembly of balls whose interior components have a much greater range of density, thereby offering a new ball choice that, in physics terms, involves the moment of inertia of a solid sphere. Eventually, "dynamic balance" regulations had to be adopted.
Another effect of ball imbalance (either static or dynamic) is the ability to introduce gyroscopic effects on the rotation. The component of imbalance along the rotation axis provides a leverage that can change the orientation of the axis on its horizontal plane, an action physicists call precession. It is basically the same thing as a spinning toy top "going around in a circle." In the case of a rotating bowling ball, as it moves along the lane, there is only time for its total rotation axis to move along a short arc, but this is enough to reorient the total rotation so that some of the forward-roll becomes side-roll, increasing the side-roll provided in the bowler's delivery, thereby achieving more hook. It is possible to use dynamic ball balancing to achieve a stronger gyroscopic effect than static balancing alone.
The advent of dynamic ball balancing meant that bowlers could achieve "ball flare" without the need for a 3/4 roller delivery, and more hook. Additionally, balls with covers that create higher friction, such as "particle" balls, provide for more traction and hook. Bowlers are embracing these choices, buying balls whose characteristics complement or enhance their deliveries.
