HP Deskjet 6122 Color Inkjet Printer - Manual (Page 26)

Do not press the buttons on either side of the duplexer when installing it in the
printer. Use these buttons only when removing the duplexer from the printer.
printing with the duplexer
For instructions on printing two-sided documents with the duplexer, see two-sided
printing .
supported paper types
The duplexer only works with certain media types. For a list of supported media types,
see two-sided printing accessory (duplexer) specifications .
2. Insert the duplexer in the back of the printer until both sides snap into place.