Something I should mention because it can be important in pool but is not covered in the material I linked to: While an internal focus generally negatively impacts performance and learning, this only applies when the focus is on what you are trying to do - on the stroke. For various reasons I won't go into, there is no problem in retaining some focus internally on something that is not, or should not be, involved in the shot.
So attention to keeping your head still when you deliver the shot is not hugely destructive, and may be helpful to someone who has a habit of moving on the shot (although an external focus, staying down to watch the cb bounce off the ob is probably to be preferred if it works for the individual).
Similarly paying some attention to keeping unwanted movement out of the wrist and shoulder can be useful.
