Thomas Cleary comments:

Some translators do not grant any semantic value to the reply No, but render it as if the master in the story had answered with an inarticulate utterance, which they gener- ally say is Mu, a Japanese pronunciation of the Chinese wu. This reflects a practice of using this meaningless syllable a kind of mantra or concentration spell thoughts and detach it from the world of objects. The original saying was in fact articulate and meaningful, and its original use in meditation was not as a spell. The mental and verbal repetition of Mu seems to have been invented in Japan around the year 1900 by monk who used it to popularize Zen, which he saw to be on the verge of extinction and in need of emergency mea sures. Relics of that movement later produced attempts at popularizing "instant" or "jet-age" Zen by throwing people an eccentric into trances, or iving them to distraction, in highly pres- sured intensive sessions lasting several days to a week or more at a time The general drawbacks of this technique are those of all incantational practices, deriving from the dangers inherent in its effect on unprepared minds. The specific flaw of the Mu repetition practice is that it tends to produce feit experience of emptiness, one that is only an altered state and does not cut a counter- through the root of the ego. As the Buddhist giant Nagarjuna wrote, "Emptiness wrongly seen destroys the weak-minded, like a mishandled snake or a misperformed spell." I have written at length about the first story because of the importance given to it by Wumen and later Zen tradi- tion. In particular, I have reported the traditional warnings of the masters concerning misapplication of this koan be- cause classical lore is full of these caveats, and because such abuse still happens. Today selves and others by malpractice of this koan, believing themselves to be Zen masters, and trying to teach others to some people still deceive them- do likewise.