The knuckleball is probably the most mysterious of baseball pitches, surrounded by a great deal of mystique. It is usually thrown at a speed significantly lower than that of “ordinary” pitches and with very little spin. The lack of spin means that the knuckleball does not experience the Magnus force that is responsible for the movement on ordinary pitches. Very early in the PITCHf/x era, we learned that the spin-induced movement of ordinary pitches bunches into relatively small clusters, with the size and location of the clusters—along with the release speed—serving as signatures for a given type of pitch thrown by a given pitcher.
Alan Nathan uses PITCHf/x to "Distinguish Perception from Reality" while answering the question — Does the Knuckleball flutter? I would actually challenge the word "perception," because even sitting very close to the field it's difficult to see a knuckleball moving.
It's the journey, not the destination, so I will go ahead and reveal the answer. Knuckleballs are "free to flutter and zig-zag" within the confines of a smooth arc, and that movement is rarely more than one-tenth of an inch in any direction. But the combination of the mystique of the pitch and the tenth of an inch is enough.