To answer the original question, english isn't meant to alter the trajectory/spin of the object ball. It's primarily used to control what happens to the cue ball after it strikes the object ball. It has some effect on the object ball, but the answer to that question is complex, situation-dependent.
When a very good player is on his game, he's said to have the cue ball "on a string," controlling where it goes at will.
And that's the fundamental flaw of Yahoo pool.. most of the high-point players have no concept of strategy or setups. They just knock down straight shot after straight shot w/ a ruler.
If you ever watch competitive 9-ball on ESPN (I know this is pro-level), notice how all but the last shot are usually not straight shots.
Pros do this on purpose.. they need an angle on the shot to control where the cue ball ends up.
English in Yahoo Pool works similarly to real life, except that it's a simplified model.