Just noticed this feature for the first time. Used to be that if you wanted to use a specific frame for a thumbnail, you had to navigate to that frame in the video, copy the timecode (or memorize it), and then paste it into the Start Time for the media's Thumbnail Property. Now I see that you can navigate into an item, find the frame you want to use, and then right-click the item (or any item, really) in the Project Overview window and choose "Insert Thumbnail from View." Handy, though I don't use thumbnails that often...

Rob