You newbie its from twitter comments box, probably page 4 of comments or something like that, loaded inside page but not showed, user must go to page4 to see it... But how to get inside that box, that is a mistery
maybe this article can help:
http://corp.topsy.com/2009/11/03/1st-birthday/ to understand plugin there...
They "probably manipulate somehow" that similar plugin on HP to show their link there...
So you need to know how plugin works to get there...