Here is a test for anyone thinking that their stroke is as straight as O'Sullivans. Set up a long, straight, diagonal shot straight in to a corner pocket ( preferably on a snooker table) but a 9foot pool table is ok too. Shoot a hard follow shot and follow the object ball into the pocket with the cueball (scratch in the same pocket). How did you do? Even if you place the cueball fairly close to the object ball and aiming is no longer an issue many people will still struggle, especially with power shots. On a snooker table, because of distance and pocket size, the difficulty is amplified many times, yet snooker pros can do this regularly.
Got a video of any snooker player doing this regularly?
I agree that being able to do it is indicative of the following things, accurate aiming, being able to hit the cue ball accurately, and a straight stroke.
This is one of the things that you do see great pool players being able to do a lot as they will often set up these shots and draw the cue ball back into the pocket below them which essentially is the same feat.