http://www.youtube.com/watch?v=n2VYJ_uIxyg
from the end of the backswing, to about when the ball gets to the baseline, the arm/torso relationship is the same...... then she stops the torso and let the arm go forward by itself.
pause the video and go frame with the right arrow on the key board.
so the arm never had to 'catch up'.... it is always in sync (by design anyways), then at the last moment it goes ahead of the core.