Early in our pool educations we learn to pocket a ball as shown in the diagram's Shot A. Here we see a ghost cue ball frozen to the striped object ball so that the line of centers, the solid line, goes directly to the center of the pocket. And we learn that making a shot depends entirely on meeting up with an object ball in that fashion. Many books teach this method, and a few companies sell high-tech devices that help us learn it.
One small problem with the idea is that, stated simply, it is mostly false. In a nutshell, making contact on the solid line only works to pocket straight-in shots-that is, in the absence of left or right-hand English. And since most instructors, regardless of experience, encourage students to avoid English early on, too many beginners suffer needlessly with missed shots.
For shots that are not dead, straight-in, the vast majority of pool shots in other words, the cut angle demands a different contact point on the object ball, depending mostly on the direction from which the cue ball travels. In Shot A, if the cue ball came from the X and hit the striped ball on the solid line, unavoidable friction would come into play, enough perhaps to move the object ball past the pocket to the X on the bottom rail. Likewise, the same, solid-line contact point on the striped ball, with a cue ball coming from Y with no English, would move the object ball past the pocket to the left and toward the Y on the short rail. Another way of stating the phenomenon is that, when we cut a ball with no English on the cue ball, we must cut it thinner than the conventional, ghost-ball method dictates. We see that illustrated in Shot B, where to pocket the striped ball with no English, you would have to hit it as shown, with the line of centers moving away from the pocket. Over cutting the object ball to the right allows the friction between the two balls to move the object ball to the left and into the pocket.
Many pool players do not want to know about different contact points and instead nurture a false belief that grows from adhering to the picture we see in Shot A with the solid line for any shot at any angle. How can a player believe something that can easily be proven false and still make balls? The answer is to introduce outside English, or English away from the direction of the cut. We know that left-hand English throws an object ball to the right and right-hand English throws an object ball to the left. So, picture a cue ball in Shot A coming from X, with right-hand English, and making contact with the striped ball on the solid line of centers. The right-hand (outside) English throws the stripe left while the friction from contact pushes it toward the right. In some cases the two forces offset each other equally to make the conventional picture work to pocket the ball. Because it can make a false belief work, outside English exerts a strong hold on many players early and can make some of us afraid to shoot a cut shot without it.
Not only do I understand how offsetting the influence of friction on the object ball can make pocketing balls seem easier, I tend to apply outside English on many cut shots in order to "help" them to the pocket. However, becoming addicted to its use for all cut shots can be dangerous as shown in Shot C. Here we have a simple cut on the first solid ball into the top left corner with natural position for the second solid ball into the bottom left corner. Players who must use outside English to pocket balls face a very touchy problem with this shot, especially on a table that plays a little fast. Because of the slight cut angle, the cue ball will bounce off of the object ball with controllable speed, making a short bounce from the top rail easy enough for a good shot on the next solid. However, the almost full hit on the object ball helps the cue ball retain more spin. So, when the cue ball hits that rail with more right-hand English than anticipated, even at a slow speed, that outside spin can easily make the cue ball spring off the cushion and travel too far for a good shot on the next solid as shown with the dashed line.
Even though outside English can seem to make pocketing balls easier or at least more reliable, it comes with some complexity that we must learn to manage. We should also keep in mind that, while it does help to counteract the shot's friction, it still demands a precise hit on the object ball as well as the ability to predict its effect on the shot and then compensate for that effect. What we must do is learn to pocket balls at all cut angles with all possible cue-ball hits to make the shots and play position in any situation. We must not let a stubborn belief lead to an English addiction that controls our game.