I was wondering about that "one hand clapping" phrase and found this: http://www3.tky.3web.ne.jp/~edjacob/koan.html . Which, so to say, didn't solve it for me. But it's an interesting text.
Initially, I thought that it means that you know that some effect or something valuable or so is hidden or generally inaccessible and you gain from it just by knowing (or believing) that it's there (or "somewhere"). But I didn't get a sufficient answer from the article. Please tell me what it means.
Technical remark: It should be written kouan, see it's hiragana.