[Damian]

Here is what we are taught.
Assuming you are lead.

As ball moves away, close down.
As ball moves to opposite side, it may dictate that you rotate over.
As you cross over, position yourself at a 45 degree angle back across the paint.
If ball comes below the free throw line extended, face the court directly. This shows that you are "on ball". When you do this, your partner should move more towards the center as he will have "off ball" on the backside which is partly in your original primary area.
Once the ball moves out past the free throw line extended or a shot is attempted, turn back to the 45 degree angle to show you are "off ball".
When transition occurs, make sure to rotate back over to your correct side.

Even though the mechanics books says to do this sparingly, we are taught to do it as often as necessary.