"Wormholes" are what I call crosslinks between two or more threads, often with click-links back. Over on Handguns you'll find a new "Tactical Mousegun" thread that has a jump to an existing thread on the subject on the Bladeforums "Tactical" forum. On the "Bladeforums" side there's a link back.
In this fashion, threads can span forums on a single UBB site or even span multiple sites. I know there's cooperation going on between BF and TFL already, so this kind of fun shouldn't be controversial? It's also useful if a moderator wants to redirect a thread over to, say, politics because it really doesn't belong where it starts yet you don't want to "kill the conversation", the answer is to start the new thread then redirect the "wrong one" that way.
Anyways, it's a "cool tool" and simple HTML. In case anyone doesn't know, you can hit "edit" for any post that contains the trick and extract the obvious-to-use code, you just won't be able to save changes unless it's your post. This works for everyone, not just moderators.