We all understand about the gearing effect when one ball makes contact with another ball at grazing incidence.
The same effect happens when a ball hits a rail. Running English is simply the gearing effect applied to rails.
If the ball has outside english when it hits a rail, it looses significant energy, and reflects at greater angle than incident.
If the ball has no sidespin, the rail will impart some on first contact and the ball will slow down.
If the ball has too much sidespin, the ball accelerates off the rail (and looses some side spin because of the acceleration).
If the ball has just the right amount of side spin, the ball reflects <I will just use the term> "well" not accelerating, or decelerating and picking up spin where angle of incidence = angle of reflectance.
You can aim the shot anyway your targeting engine has decided works for you.
I can use little more or little less sidespin* to move CB around interfering balls and make 3,4,5 rail shots with interference on the table.
(*)Altering point of contact (i.e., aim) to compensate for the different possible paths.}