Billiards TheoryThe 90° Stun Rule
If the cue ball is stunned onto the object ball (skidding on impact) it will leave the object ball at a path that is approximatedly 90° from the path that the object ball takes. This is true for all object ball contacts.
Application of 90° Stun Rule
This rule can be useful in determining whether a loser or a cannon is on. In other cue sports is is used to determine whether the cue ball will possibly enter a pocket after contact.
One of the most useful shots that employs this rule is the stun loser into a middle pocket, often used after a middle pocket loser when the object ball does not come back enough and is short for the next loser. On first glance, this shot looks difficult, but if you set the cue ball correctly in the D and get stun on the cue ball, then the shot is almost a certainty.