Billiards Theory90° Screw Rule
If the cue ball impacts the object ball at the half ball contact and with a significant amount of backspin, it will leave the object ball at a path that is approximately 90° from the cue ball's orginal path.
Application of 90° Screw Rule
The most common shot that uses this Rule is the screw loser across the table . In this case, you need to get the screw on the cue ball, whilst hitting the object ball at half ball and the cue ball will travel along the cushion to the pocket. If the shot is played thicker or thinner than half ball, the shot becomes much more difficult as the Rule does not apply.
On an average table the angle will not be 90° but will be closer to 83° (when coefficients of restitution and friction are taken into account).