As with other pivot systems, this one is sensitive to distance between cue ball and object ball. Here's a drawing that shows why:
View attachment 76999
The red line is the 90-90 line. The black line is the cue stick, pivoted at a 10-inch bridge length to the center of the bottom white cue ball. The blue lines are the cut angles you get with yellow object balls at different distances.
You can see that the cut angles get thinner as the OB gets farther away. After a certain distance (about 28 inches for a 10-inch pivot) the CB misses the OB entirely. So this 90-90 pivot only seems to work for relatively short CB-OB distances, where the changes in cut angles are the greatest for small changes in distance.
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