Matsui said:
Curious too. How does it work?
AI's answer: Find your dominant eye, hold your putter with the shaft perpendicular to the ground, and stand behind the ball with your non-dominant eye closed, letting the putter hang to cover half the ball; observe if the hole appears left or right of the shaft to gauge the break, aiming for the gap or side where the hole disappears, indicating the slope, using it as a rough guide for the putt's start line.
My experience on the practice green: It worked pretty much as the above paragraph explains but it definitely takes some practice. Once you get used to how you see the gap or line that is revealed and how it works in reality you will gain confidence. You need to make sure where you're holding your putter is the correct point where it will hang perpendicularly and not be at an angle. Putters are weighted differently and will hang at an angle if held "incorrectly". You can check this by holding it up to something you know is perfectly perpendicular like a door frame.