Since this is a hard concept to understand, let me try to get there by a slightly different path....
You are playing one pocket. You have long bank straight back to your pocket. The cue ball is near the foot rail. You want to play it just hard enough to leave the object ball in the jaws if you miss. Also, you want to freeze the cue ball on the far cushion for the safety, limiting your opponent's possibilities.
Let's suppose the object ball is even with the foot spot. Can you shoot the object ball at the required speed and also freeze the cue ball to the far cushion for a safety? No. It's impossible. The cue ball cannot roll that far for that speed of the object ball. (An exception would be if the far cushion is horribly dead, but let's stick with normal rails.)
Let's suppose the object ball is a diamond from the head rail. Can you get both speeds right with a rolling cue ball? No. It's impossible. The cue ball is guaranteed to bounce off the head rail if you have enough speed on the object ball to get to your pocket. (You might be able to do this shot with a "stun run-through" (partial follow, not rolling smoothly), but that's going to be very, very difficult to control over that 6+ diamond distance.)
For one case, you have not enough speed on the cue ball and for the other case you have too much speed on the cue ball. Somewhere between those two cases, when the object ball is roughly even with the side pockets, it's not only possible, but if you have the intended speed on the object ball, you are guaranteed to have the correct speed on the cue ball.
