Post turns - comparison
1) Because I would like my dog to already know they are turning as they prepare their body to jump- much earlier than you can turn your shoulders away (unless you intend to send the dog to jump #3 with your behind).
2) In the process of rotating, I show my dog all kinds of paths that are not the one I want. Meaning, I first show the point "Y", then "Z" and then finally the "X" on the way to jump #4.
Most dogs do not wait to see if you are going to continue rotating or not, they act on the first new information they get after they commit to jump #3.
3) It delays what the dog really wants to know. If you turn and then set the line, the dog has no idea when the rotation is going to stop, 10 degrees, 90 degrees, 120 degrees... If you set the line first and then rotate, you have given the dog the critical information first.
4) It can bog you down. While you are turning, you aren't doing something else that could be very useful to the dog.