That's an adjustable hemmer. There are instructions for using it in pdf format on the web.
Oops, sorry, forgot the link:
http://www.ismacs.net/singer_sewing_...uals/15-91.pdf
The adjustable hemmer is on page 45 of the pdf - page 43 of the manual. :)
They're far too complicated to mess with, for me. I'd rather press and fold and press and sew. :)