Well, I'd think if the CB went left/right with no side spin, you need to adjust your alignment. If the CB stayed pat while spinning in any direction, it shows the player that his/her tip placement is off. I'd guess some players could have a combination of the two, showing the teacher that a few things need to be adjusted.
Nevertheless, getting feedback from the above will not build a stroke. Stopping a CB dead in it's tracks doesn't mean you have a "developed" stroke... it just means you stopped the ball dead in its tracks. That's what I meant as far as it not showing you anything. I'll agree to disagree that stopping a ball perfectly means nothing as far has "having a stroke." I can execute PERFECT stop shots 1-handed... does that mean I have a pro-tour stroke? Um, nope.
I think you'll find that those who can execute a replacement shot successfully on a 9'er more often than not compared to the next person who really can't --- the person who can't is likely the underdog in most pool games.
Why -- because the person who can has a more refined, developed stroke.
I think that's all I have to say on the topic -- but I totally respect everyone's views on here. For once, DCP started a healthy conversation.