I've always thought this was one of the hardest koans in the Mumonkan. Even knowing the basics of foxes - long-lived, nigh immortal, supernatural trickster magical beasts - doesn't help much at all.

And even when you think you perhaps understand the causation bits, you still have no idea what on earth that Persian stuff is about!

Counting on Wumen's commentary to clarify things? Well, he makes the causality a bit clearer, but leaves the rest as mud:

`The enlightened man is not subject.' How can this answer make the monk a fox?

`The enlightened man is at one with the law of causation.' How can this answer make the fox emancipated?

To understand clearly one has to have just one eye.

Controlled or not controlled?
The same dice shows two faces.
Not controlled or controlled,
Both are a grievous error.

(Incidentally, no hat tip is merited. That page is rather incomplete.)

EDIT: turns out I'm not the only one who thinks it's unusually hard. Wikipedia has:

"The meaning of the kōan has been the object of intense debate and scrutiny within Zen due to its complexity and multi-layered themes. It was rated by Zen Master Hakuin (1686-1769) as a nantō kōan, one that is "difficult to pass through" but has the ability to facilitate "postenlightenment cultivation" or "realization beyond realization" (shōtaichōyō).[1] Important themes include causality (karma in Buddhism), the power of language, reincarnation, and the folklore elements involved in the insertion of the fox into the tale."