The infamous pageflip : Adding a page parker system

Hi everyone
Please check this link :
http://www.castorama.fr/satellites/2006/0607_s30_catalogues/index.jhtml?_requestid=66326
Once it’s open, clik on one of the books or on “consulter le catalogue en ligne”…
Once this is done you should be on an interactive catalogue that uses pageflip…This my question :
See the post-it block in the middle of the pages on top? It’s used for marking pages…HOW DID THEY DO THAT???
I can’t even start to figure out what technique i should use…
Does anyone have the slighest idea?