Forking a thread quotes a post and begins a new thread in the same forum. This is most useful when a thread meanders from the OP's topic. This is, incidentally, what the "Fork to New Thread" button does.
Eventually I want all the buttons to look like it to reduce the graphic load on the server - the "button" is actually created by css. For now I just want it in and working. Since it's a new feature having it be different makes it stand out.