On ur current page the information you want must be set up DIV or some other BLOCK element. Blah Blah Blah, yes im boring you with all this crap. But anyways you grab a bit of code from the reading source. Afterwards you compare that information to the current information or last post made on you webpage. If the information is different it means a new post was made and it should fetch that information from the database.
1 Thing i forgot, you have to set your HEADERS to not cache the page, if not AJAX might not load correctly.
Thanks for the good illustration of the working mechanism, but I have a question why did you explain it ? as I think admin knows it well and me too. So i would really appreciate it if you have a better code than the admin has provided.