There are good defensive players who try to get us out of our rhythm, forcing us to take legitimate shots off the rail just to avoid fouls. There are also times when we make mistakes in preparing for the next shot where the object ball becomes completely obscured. It is hard enough to make a legitimate shot using natural angles off the rail. But what if there is no natural angle? English will help us estimate where the white can divert from its natural course if a parallelogram (see "Of Parallelograms..." article) is not available.
The idea is to get the most out of the available angle and apply an estimated english. First, draw an imaginary line a natural shot will take the cue ball to. Then apply english. But how will you know how much english to apply? Only experience will take you to a good shot.
Here is a good exercise with the premise that the stroke has a soft follow through for a rolling shot, and not a stun shot nor any other forceful stroke. Put the white in the middle of the table directly in front of a table marker. Aim straight to the marker with varying english, starting from the center of the ball going out wide. Remember to always chalk before taking exaggerated spins, and do not be afraid to hit the outer edge of the white. Do this over and over (left and right spin) until you get a good idea where different degrees of english will take the cue ball to. Then hit the rail at an angle with varying degrees of english, both left and right spins. But before hitting an angled rail, first mark where the cue will travel with a natural shot. Then remember the deviation. The wider the english on a favored angle, the less force is needed because the white will travel faster. On the other hand, the wider the english on a reverse angle, the slower the cue ball goes out of the rail.
This is why pool is such an interesting game. Majority of the aspects in the game requires precision, but there are many more aspects in the game that require estimation… and it works!
