This customisation allows your visitors to hover over the thumbnail for a gallery and see a preview of the newest 5 photos in that gallery. Please try out the example galleries below by hovering your mouse over them (you must hover for a half second to start the preview).
Examples, hover on me!
If you were able to tweak your YUI configuration, you might do something like this to it:
You also need to add this CSS to your theme to stop things shifting slightly on Chrome when the tiles are fading: