No, this cheat code you enter when you are booting knoppix as a kernel parameter (i.e. you write in syslinux: "knoppix no3d"). More about cheat codes: http://knoppix.net/wiki/Cheat_Codes (this particular one is actually not listed there, if I see correctly). Enjoy!
To everybody - am I doing something wrong if "standard" key-shortcuts like <Alt>F1/<Alt>F2 does not work for me after replacing compiz with metacity, or there is some bug?
BTW, if you were interested in a small linux for hacking, I'd actually recommend grml instead of knoppix.
OT: I also registered just to write you the reply.

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