Apple Aperture Getting Started - Manual (Page 168)

168 Chapter 16 Using the Light Table
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.