Slideshow Tracking makes it easier to keep your place while exploring an image gallery. On closing a slideshow, or returning to the thumbnail index from a single-image page, the page will scroll to the location of the corresponding thumbnail for the last image viewed and will mark that thumbnail with an icon. Clicking the icon allows you to resume the slideshow with that image.
This is an original feature that we dreamed up during CE3 development, and we're very proud of it. Why more galleries don't do this, we fail to understand. We are extremely pleased to have brought it forward into CE4, though.
You may disable slideshow tracking in the Appearance pane, under Slideshow Tracking, though we can't imagine why you'd want to do that.
The Turning Gate
Creating webbly, wobbly, Lightroomy things since 2007.