Apple Aperture 2 - Manual (Page 530)

530 Part IV Distribution and Backup
To reveal images in the Light Table that are covered by other images:
1 Select the top image in the group.
2 Click the Uncover button (or press Shift-X).
3 Select an uncovered image.
The images are regrouped, with the selected image on top.
The top image is
selected.
Click the Uncover button
to reveal all images under
the selected image.