Koans are a mysterious and diabolical device for drilling down through layers of delusion, confusion, personal strategies and karmic tangles to arrive at an experience (not an idea) of a stunningly simple and fundamental truth. The goal is transforming, not collecting.
Training with koans is like a game of pin the tail on the donkey, only in this game, you are the donkey, who is led blindfolded into a pitch-black room, spun around and told to go at it. You stab and stab again, bump off walls you cannot see, fall down and pick yourself back up and keep on groping. when you're exhausted and completely off guard, a pin appears from nowhere and punctures you.
A koan is like a Chinese puzzleball made of opaque glass. You know there is a way to the center, but as many times as you turn it over in your hands in any kind of light, you cannot see a seam or feel a crack. you discard the ball in frustration, but then you find that it has worked its way into your hara, your lower belly, where i sits, ticking away like a time bomb in the machinery of the constructed self. It bides its time, waiting for the mind to be at utter rest. Not a single human thought. then an unexpected jar or jolt--it could be the slightest thing, the sound of a key on a table, a breeze, or the sight of a single flower petal--and suddenly it explodes. It takes a while before the mind realizes that something has occurred.
You never know what will trigger the explosion. You never know what piece of delusion of "I, me and mine" it will take out. Koan work is like psychic surgery. After it is over, the surface is intact, but something deep and diseased inside has been removed.
A koan is not a problem you can "solve" with the intellectual mind. Koans bypass the mind in order to solve the problem of "you." Koan work is a lifetime process. You don't ever finish one of the old koans.
--Jan Chozen Bays, Shambhala Sun, March 2004