In a very rough nutshell:
Core numbers are not the only thing about a ball. Most of its reaction comes from the coverstock and its preparation. The core is just a "motor" inside that can be tuned to save and deploy imparted energy in a certain way. The RG numbers give a vague indication what the core is capable of - but finally, you have to look at the whole cover/core/condition/player type picture.
RG:
Low numbers mean that the mass is "grouped" in the center, close to the rotational axis. The more mass is grouped in the center, the easier energy can be imparted to the whole ball and the quicker it is also bled, e .g. when the ball encounters friction. This can be both beneficial or a thing to worry about - a low RG is generally handy on long oil when you want the ball to roll even with low friction, a high RG can be useful to save energy for the back end.
RG differential:
The difference between RG min. and max., measured at an axis through the pin and the core center. Through instable rotational pin placement off of the preferred rotational axis of the core (PSA), which is at 90° to this pin axis, you determine how much the ball flares through the rotational forces which get the core back into its PSA. Sounds abstract, but that's what pin placement is for - the more instable the core, the more flare you create, which means the core has a longer way to move towards its PSA, and the stronger these forces work once the ball is rotating.
With flare, new/clean coverstock is exposed to the lane and creates friction, the ball depletes energy and once the ball is rolling at its PSA it has lost a lot of its "steam". an instable pin position can mean a lot of early hook, but also lots of energy depletion - doing too much can easily become detrimental for the ball's effectiveness!
RG asym. diff:
Similar to RG diff., but measured at 90° to the former axis, actually through the ball's PSA. The stronger/bigger the number, the more the ball wants to migrate towards its PSA - it makes the ball's reaction more defined, and from a certain "strength" on (say 0.01" or more) it is a force which actually can be exploited to fine-tune a ball's reaction. Again, this is neither good nor bad - it is IMHO an option if you exactly know which breakpoint shape you want, but you might sacrifice versatility.
Hope this helps - certainly not complete, but maybe it helps understanding the forces at work and how the numbers relate to them. Just remember: the core is not everything - look at the whole ball package and what you want from the piece.
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DizzyFugu - Reporting from Germany
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