It is a partially manual function - you can choose to print both sides of your document, and then the printer prints the odd number pages first and prompts you to simply take the set of copies and put them (same side up) into the paper tray and you hit the OK button to finish the duplex. its not quite as easy as a full function duplexer, but its better than trying to duplex yourself on a regular printer, especially if you have more than two pages.
I called HP after I saw the same thing in my specifications that came with the printer, and found out that if you want the full duplex function, you have to purchase the next model up - the "M477fdw" (I think these are the correct letters) - the d is for duplex. You will pay up to $100 more.