He placed the ball in the hole of the CD to use it as a mirror, to check if he were exactly over the ball and the head didn't move during the stroke.
I have used a CD (thanks AOL) for several years in putter fitting. I have improved this device by scoring two lines on the mirror. The lines are perpendicular to each other. One line shows where your eyes should be and the other shows putterface alignment. After scratching the lines with a sharp scribe I fill them with a sharpie. When your eyes are in the right position I mark the grip for shortening. I can twist the grip so that the putterface aligns square when the golfer holds the putter with a "comfortable" hand position (for him/her). Contrary to most I believe that not everyone can rotate the hands/wrists to keep the face square. Each person seems to have a nonstandard "body geometry
Arnie
