Do you know the exact path the cue ball will travel? If you understand the tangent line, then you can visualize the cue ball path much easier, and allow you to position for the next shot with more precision.
The tangent line is an essential component when learning to position the cue ball. Sometimes, even advanced players needed to be reminded about the basics.
When the cue ball collides with an object ball at an angle, the object ball will depart on the line connecting the centers of the two balls, and the cue ball will travel down the tangent line. The tangent line is 90 degrees from the line connecting the centers.
If the cue ball does not have any top or bottom spin when it collides with an object ball, then the cue ball will travel down the tangent line without deviation. This is known as the 90-degree rule, and this type of shot is called a stun shot.
If the cue ball collides with an object ball straight on without top or bottom spin, then it will stop in its tracks. This type of shot is called a stop shot. Stun and stop shots are the most consistent shots you can use for positioning.
If the cue ball has topspin, then the cue ball path will bend forward from the tangent line when the spin catches (after a delay), creating a follow shot. Similarly, if the cue ball has bottom spin, then the cue ball path will bend backward from the tangent line, creating a draw shot.