It would be nice to write another article that explains why it works. Add labels like 0, 1, 2, ..., 6 to the nodes and explain that black arrows are +1 and white arrows are x3.
> How does it work? Here's a hint: the white arrows correspond to 10x mod 7.
(which of course is the same as 3x mod 7), it seems that the author intentionally wants to leave it as a slight puzzle. But blacksqr's suggestion (https://news.ycombinator.com/item?id=27222819) to write it yourself is even better!