When you roll your ball down the lane, particularly an oily part of the lane, your ball comes back and has oil rings around it.

Those oil rings are in indication of where the ball coverstock rolled through the oil.  By measuring the widest distance between the rings, you get Track Flare.  The wider the Track Flare, the more fresh coverstock on the ball is digging into the lane and the more aggressive the ball is.  A hook monster will have a large track flare, but your plastic spare ball will have a very small one.  

Hopefully this explains it.  There is always a bit more, but this is a general idea.
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D <~~~~ Used to be terrible wiffing 10 pins.  Now through much practice, can wiff any single pin spare at any time.

Darrell Guzman
guzmand19 - Yahoo IM and MSN screen names