It is a crop from the original image (shown in the book but I couldn't find a picture of it). In that, the woman is centered in the scene and the angle of her hips is on an implied diagonal from the bottom left to the top right.
With a bit more searching, I have found the image in question - it is called "Clothed Nude" and is by a photographer called Nic Tucker. The version shown on his site is higher contrast than the one in Cheung's book and doesn't look as good but it does illustrate better how the woman acts as a lynchpin for the skewed horizon. Click on the photo for a link to Nic Tucker's site:
Wulf