Remember Spirograph? You put the point of a coloured pen into a hole in a plastic wheel, then rolled that wheel round the inside of a plastic ring to make a pattern. The two bits of plastic were held together by cogs.
Well, this is a similar principle. I disengage the feed dog, which moves the fabric in a straight line through the machine, fit this foot, and then off I go.
The notches round the foot rotates it as the stitching proceeds, so the stitching forms a circle. All sorts of embroidery stitches on the machine can be used to get different effects.
The size of the circle can be varied by adjusting the position of the tongue that goes across the middle of the foot. If you forget to tighten the screw, the vibrations of the machine move it gradually to the other end of the scale, making a spiral effect. Accidents are sometimes useful.
There is lots of fun to be had with this toy, although I will have to remember to stablise thinner fabrics. This piece of calico was 9 1/2 inches wide before I started to stitch, but the sides both shrank in by about 1/4 inch after I had stitched it.