Ouch, my description was wrong...it tells you to do a 360 degree rotation of the lines and not a 180 degrees rotation, sorry. With a 360 degree rotation of the lines someone can create a cutting object* for a split torus model (Keizo Ushio stone sculpture)...;-)
*Simply cut a torus with it in two halves and then arrange the parts shown like shown in the picture.
splitorus.jpg
