That's only the Rouleaux bit. The article details a bit that produces sharp (vs. rounded) squares.

It was confusing b/c the pictures didn't show that the bit's axel had to orbit independently of the bit. This animation cleared that up for me even if it wasn't the exact case.

The link above is an animation of the Rouleaux Triangle and will draw a square with rounded corners. In this article, the author talks about a variant of that triangle that will draw exact squares.