The 30 degree rule can also be used to help predict where the cue ball is going to go after object ball impact.
Unlike with the 90 degree rule, which apply for a stun shot, the 30 degree rule applies when a cue ball is rolling at object ball impact, and this is fairly common for the cue ball to be rolling. So when a cue ball hits the object ball at cut angles, that are fairly close to a half ball hit, where the cue ball is heading straight for the edge of the object ball, but again there is a fairly large range where this applies. When that happens, the cue ball's angle will be deflected by 30 degrees, so the cue ball is originally heading this way along this line. After impact, it will be deflected by 30 degrees along this line.
Your hand can be very useful in planning this, a firm but relaxed V sign will be very close to 30 degrees. In this case, since cue ball is fairly close to the object ball, I will hit with slight follow or top spin to ensure that it is rolling at impact.
As you can see the cue ball followed the 30 degree parallel almost exactly.
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