On these kinds of shots, you can use sidespin to control the behaviour of the cueball and the 8 ball, especially at low speeds. I've wanted to try to explain this in my thread about creating breakballs, but it's very complicated to explain (but easy to show on the table). If you are hitting the ball kind of full, you can use left spin to push the 8 ball down and right spin to hold it (push it more up). Naturally using left spin will also lead to the cueball running past the 8, for more angle, while the opposite spin will tend to keep the cueball from running (slowing it down). Left spin in this case becomes "outside spin" when the cueball hits the 8, even if it's inside when playing the 6. It's kind of complicated, because you have to consider both the behaviour of the 8 as well as the cueball.
Let's say you have this shot and you see that at the angle you have, the cueball is going to run way past the 8 after hitting it, bouncing out off the rail, and probably leaving you no angle, or the angle opposite of what you want: Try to hit the shot slow with right hand spin, and you may be able to hold the ball. For the opposite situation you use left spin. Heavy left spin will also help you a bit on certain angles because (obviously) when the ball hits the rail it will go left at a wider angle, keeping it closer to the rail than a no english shot. If you hit it like a gorilla, this does no longer apply. You can increase your margin of error on the shot this way, using the slow spin to give you a flatter cueball line along the rail.
It's very hard to show these things to certain people, because they lack the feel and the ability to hit the rail at the slowest possible speed with a lot of spin. They are going to muscle the ball with a rigid death-grip over and over and then say it's impossible. It also doesn't help that they have been told that english only affects the balls rebound angles off rails and not off balls. They don't realize that at slow speeds the sidespin becomes very important for the behaviour of the balls.. Try to explain these kinds of shots to a snooker player, unfamiliar with pool rails and who constantly slams every shot in...That is definition of frustrating! I tried to teach someone a delicate shot like this not to long ago, but I had to give up, because he had zero feel and finesse on soft shots. He shot straight, and he did understand what I did when I shot it, but he was unable to replicate it.
Hope these explanations are at least somewhat clear (I tried).