If you briefly cross your eyes you can:
1) level your head to read the green and
2) align your eyes with the line of your putt.
Yesterday I was trying to read the practice green at the driving range. The green has a lots of subtle slopes, and I kept reading a break to the left where it was actually to the right. On a whim I crossed my eyes as I looked at the hole. This produced two images of the little flag and the hole and they weren't level! One image was slightly below the other like this:
HTML Code:
|>
| |>
| |
O |
O
I tilted my head until the images were lined up
HTML Code:
|> |>
| |
| |
O O
and I was able to see the break correctly.
Apparently the line between my eyes was not perpendicular to the perceived gravity vector. In short, my head was tilted.
I tried crossing my eyes again when I looked down at my putter as I addressed the ball. This time the two images of ball and putter were out of alignment with the line of the putt.