The background story of this Koan
The traditional commentaries tell about Fuketsu's ability to express his Zen "like a flash", pretty much in accord to his school's teaching of sudden enlightenment. There's no waiting, studying and cultivating of meditation insights. That's "dead Zen", taught by other Zen schools. That's not Fuketsu's cup of green tea.
But when Fukutsu was confronted with a question about the dualism of talking and silence, he failed, the commentators claim. Instead of cutting off the monks erroneousness and false dualism, he disappointedly answered in the conventional way of educated Chinese with a poem.
But, quite typical for Zen, what's criticise by the commentators is just what's excellent. The poem describes a sudden experience "truth".
Walking in springtime in nature with an open mind, you've an immediate impression of scents, sun, bird songs etc. That's what nature is. Just this sudden "impact" on my senses.
This answer immediately cut's off any reasoning about talking and silence, brightness, subtlety and so on. Being not thinking is the core of Zen.
The original Chinese Goang
Because a monk asked, “Talking and silence wade across brightness and subtlety. So how is one to flow unobstructed and not offend?”
Xue said, “I long remember South of the River* in the 3rd month
The place the mountain quail cry,
The fragrance of a hundred flowers.”
(* "South of the River" means the region of Jiangnan.)
Traditional Commentaries and .... Poems (Gata)
Fuketsu used to have lightning Zen.
Whenever he had the opportunity, he flashed it.
But this time he failed to do so and only borrowed from an old Chinese poem. Never mind Fuketsu's Zen.
If you want to express the truth, throw out your words, throw out your silence, and tell me about your own Zen.
Fengxue’s function was like pulling on lightening and obtaining an expedient path for walking.
He contended, but he sat on the tongue of the person who came before, he didn’t cut it off.
If within you are able to see intimately, naturally it happens you go out personally on the zig-zag path.
Just abandon the samadhi of language and be ready to say one line of verse when you come.