Banking Shots
Sending an object ball off a cushion into a pocket, using the mirror-angle rule as a starting point.
The mirror-angle idea
The simplest model for a bank shot treats the cushion like a mirror: the angle at which the object ball approaches the rail roughly equals the angle at which it leaves, measured from an imaginary line perpendicular to the cushion. Picturing the pocket's reflection on the far side of the rail and aiming the object ball at that reflected point gives a solid first approximation of the right line.
Why the simple mirror is not exact
Real cushions do not behave like a perfect mirror: the ball's spin, its speed, and the cushion's own rebound characteristics all shift the actual rebound angle. A ball with topspin tends to come off a bit shorter (a tighter angle) than the mirror rule predicts, while a slower, sliding ball tends to come off a bit longer. Experienced bankers adjust their aim slightly off the mirror line to account for these effects.
Speed and spin matter as much as the line
Because rebound angle is sensitive to speed and spin, consistent bank shooting depends on hitting the object ball with a repeatable, moderate speed and little unwanted spin, so the rebound behaves predictably. Many players deliberately avoid extra English on banks unless they are using it on purpose to adjust the angle, since uncontrolled spin on the object ball is one of the biggest sources of missed banks.