Present a sword if you meet a swordsman;
Don’t offer a poem unless you meet a poet.
When talking, tell one-third of it;
Don’t divulge the whole at once.
~ Mumon’s verse on Case 33 of the Mumonkan
One of the great things about the book, Two Zen Classics: The Gateless Gate and The Blue Cliff Records, translated by Katsuki Sekida, is that Mr. Sekida gives you information that you have no way of knowing if you’re studying Zen by yourself.
As just one small example of this, Mr. Sekida tells you that the line, “Present a sword if you meet a swordsman,” can be interpreted as, “Suit your sermon to your audience.” Similarly the line, “Don’t offer a poem unless you meet a poet,” can be interpreted as, “Don’t cast pearls before swine,” which I interpret to be people who don’t understand Zen, or more likely, people who have no interest in it.
In many other cases he gives you information you have no other way of knowing: The meaning of a flagpole in Case 29; background about the old woman in Case 31; the story of Seijo and Ochu; and so on. Zen koans are something you’re supposed to study very hard on your own, but if you have no idea that a flag is raised up a flagpole when a Zen master is going to give a sermon, all of your studying will be to no avail.