The NYTimes one actually comes out at the end of EVERY article. At first I didn't think it did, but I realized the article I was looking at was 2 pages. Once I got to the end of the 2nd page, the box popped out.
If "I" was going to code it, I'd probably make it slide out everytime. Why? As I'm browsing through a forum, I would probably want to see similar topics for each thread (blog?) I am reading, so if I wanted to find out more about said topic, its immediately available. If it wasn't every thread, there is a chance that the one item you want to know more about doesn't have a popout showing similar items. It appears they are strictly using it to give the reader a link to the next article (articles are based on time published), so that the reader doesn't have to go back, or scroll to the top, etc.
