Not sure where I found it, but it sia comprehensive explanation:
"Forward oil" indicates that conditioner is being added to the transfer/buffer rollers on the way down the lane, from the foul line toward the end of the lane. From the point the last "loads" are added, everything past that point is just "buffed" in meaning that no additional volume is being added.
The same holds true on the "reverse oil" except that the added conditioner starts down toward the end of the pattern and is applied coming back to the foul line.
What this changes is the "taper" of the pattern. Taper basically being how smooth the transition of oil going down the lane, not side to side which is what is checked for the "ratio" of the pattern, i.e. when you hear a Sport pattern of a 2:1 ratio this refers to the side to side amount of conditioner. There is no specification on lengthwise taper for Sport shots or any other shots.
How a taper affects ball reaction is basically that it determines how soon or how late a ball might get into its roll. For example, a pattern that has very little taper would be classified as pattern with a lot of "skid/flip" reaction where a pattern that has a long taper would be more conducive to a ball rolling more smoothly or an "arc" reaction.
If you see a pattern where most of the conditioner is applied "forward" that will typically have the ball rolling up earlier and then smoothing out on the way down the lane.
If you see a pattern where most of the conditioner is applied "reverse" then that typically is going to give you a very strong skid/flip reaction making you have to control the backend.
If you see a pattern that is pretty equal, forward and reverse, then those types of patterns are more open to varying styles of bowlers, they dont necessarily dictate that you play them a specific way."
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DizzyFugu - Reporting from Germany
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