Enter - tangent line english. If your cue ball looks like it is headed for the pocket on your next shot, you can steer it away from a scratch by using some follow or draw on the cue ball.
Once you become proficient at this type of english, you can really prevent a lot of scratching during pool games.
The use of follow on a cut shot will cause the cue ball to travel somewhat less than 90 degrees after it hits the object ball.
The use of draw on the shot will cause the cue ball to travel more than 90 degrees from the object ball line of travel. Depending on the amount of draw you apply, you can really alter the course of the cue ball.
The following diagram illustrates the directions the cue ball will travel after impact in shots where the shooter is using either follow or draw on the cue ball.
Watch the video below for a demonstration of these principles.
Once you learn the concept of tangent lines, you will have a much better idea of where the cue ball will end up after a pool shot, and you can plan your game accordingly. How much would your game improve if you could keep that cue ball from getting stuck behind another ball and ruining the next shot?
If you develop the ability to move the cue ball rebound to almost any spot on the table, you will be making longer ball runs and winning more games.
I keep the tangent line visualization in the back of my mind when I am looking at a shot and figuring out the angles. It really helps to understand how the cue ball will react and what direction on the table it will travel.
If you find yourself scratching a lot, pay attention to the tangent line and get a handle on what the cue ball will be doing during your shots. If you understand and use the tangent line concept, you'll know where that cue ball will be going.
The following video is a good demonstration of the 90 and 30 degree rules. The shooter/actor does not specify the 30 degree rule by name, but if you observe the shots where he uses follow on the shot, it is obvious that the cue ball travels approximately 30 degrees on the rebound.