It acts as part of a pulley. A cord is wrapped around it and that cord moves one of the axes. I suspect that since stepper motors had a high acceleration they may have found the cord wrapped around a rubber/vinyl pulley may have slipped too much. I worry, though, about the cords wearing through too quickly.
Building this thing is different. While you can download a PDF set of instructions, the pictures are smaller. If I use the main html instructions I can click on a small photo and it links to a larger picture (color pictures).
This seems to be the new paradigm. Many of the FSM articles these days are mainly a bunch of pictures with captions, and little text. I guess folks are worried about a decrease in literacy.