This, roughly, is what happens: Auto-Tune locates the pitch of a recorded vocal, and moves that recorded information to the nearest "correct" note in a scale, which is selected by the user. With the speed set to zero, unnaturally rapid corrections eliminate portamento, the musical term for the slide between two pitches. Portamento is a natural aspect of speaking and singing, central to making people sound like people. A nonmusical example of portamento would be "up-speak," a verbal tic common in some people under thirty. (Can you imagine the end of every sentence rising in pitch? Like a question?) Processed at zero speed, Auto-Tune turns the lolling curves of the human voice into a zigzag of right-angled steps. These steps may represent "perfect" pitches, but when sung pitches alternate too quickly the result sounds unnatural, a fluttering that is described by some engineers as "the gerbil" and by others as "robotic."

The gerbil.

Update: Now with audio! "Here Frere-Jones talks about how Auto-Tune has become a pop-music phenomenon, and demonstrates how it can transform the human voice, with the help of the music producer Tom Beaujour."