Technically speaking it allows you to keep the hands inside the sweetspot plane and turn the face to the ball earlier. This allows you to use the right arm as hard as you want and not worry about squaring the face. The holy grail for me personally (and all you underplaners)
The more out the left arm the more late roll or roll/arch of the left wrist is required. You can do this but id like to strive for more #3 accummulator roll early then right arm POW with a free wheeling clubhead.
Here's Kevin's explanation:
and this:
Nevermind
What did I do?
He wanted you to post the link. I assume he found it.