what is vertical feed?

I've noticed this feature on a couple of machines listed for sale. what is it?

http://portland.craigslist.org/mlt/atq/3467077634.html

Here's a video:

http://www.youtube.com/watch?v=1rxpWrP5Wrc

that's interesting! thanks jlhmnj. I've never seen it b4. must not have been so popular....I can see some limitations....lol

I wonder what happened to the cabinet and irons? I imagine that one of the treadle butchers did heinous things with the cabinet and irons. :shock:
This machine takes 15x1 needles just like modern machines do. It wonderful for sewing slippery fabrics and for straight line quilting and sewing binding on your quilt.
Sharon