The Jargon File (version 4.4.7, 29 Dec 2003):

nerd knob
 n.

    [Cisco] A command in a complex piece of software which is more likely to be
    used by an extremely experienced user to tweak a setting of one sort or
    another - a setting which the average user may not even know exists. Nerd
    knobs tend to be toggles, turning on or off a particular, specific,
    narrowly defined behavior. Special case of knobs.