HP Deskjet 5650 - Manual (Page 78)

Do not discard the Rear Access Door. In order to print, the
duplexer or the Rear Access Door must be attached to the printer.
2. Insert the duplexer in the back of the printer until both sides snap into place.
Do not press the buttons on either side of the duplexer when
installing it in the printer. Use the buttons only when removing the
duplexer from the printer.
printing with the duplexer
For instructions on printing a two-sided document with the duplexer, click here .
For instructions on making automatic two-sided printing the default print setting for your
printer, click
here .
supported media types
The duplexer only works with certain media types. For a list of supported media, see the
specifications in the documentation that came with the duplexer.