Last again
The "boing" concept is probably best expalined using an example. If you were to go into the Events forum and post a thread on some obscure race that no-one had ever heard of, chances are you would get no replies. At the rate threads are added on the Events forum, your thread would move down page one, onto page 2, then 3, and by the end of the day would be on about page 6. Feeling slightly miffed, you want to see your important message back in it's rightful place at the top of page one, for all to see. The only way to do this is to post something, anything, as another message. This "bounces" the message back to the top, being the thread with the most recent post.
In true Zebedee fashion, "boing" has become the "in" word for posting, when you don't have anything interesting to say.