Hey lilphyzx,
Typically, a pin up will offer you more length so if you want a stronger backend, that would be the suggested way to go. The more important part of this is where you place the mass bias because it does control the reaction "shape" of the ball.
The pin up/down however is also very relative to your speed, tilt and rotation. If you have a lot of tilt, a pin under can also work because it can help you get the ball started a little sooner vs blowing past the breakpoint. Hope this helps explain some...
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Rick Leong
Ten Pins Pro Shop
Track Intl. - Amateur/Pro Shop Staff
Vise Inserts Staff