I think bikesnobNYC noticed this a while back. Something is attached to the seat tube, if I remember correctly, and as the tire passes over or through it, the tire is covered in chalk dust. It's a nifty gadget, but I think the makers or marketers are kidding themselves if they think it's going to reclaim the roads for bikes or get more people riding. They aren't, however, kidding themselves if they think other people will think this and buy them. I think.