In plain terms — A cycle is not a perfect clock: sometimes it is wider and longer, sometimes narrower and shorter. This “breathing” is called MD fluctuation (magnitude-duration).
Definition
Magnitude-duration fluctuation is the third element of the price-motion model presented by Hurst in Chapter 2: each periodic component may vary slowly in both magnitude and duration.
It must not be confused with a modern numbering of the cyclic principles. In Profit Magic, MD fluctuation appears in the model alongside the secular trend and the sum of periodic components.
It is not a model bug—it is structural to price.
Source: J. M. Hurst, The Profit Magic of Stock Transaction Timing, Prentice-Hall, 1970, Ch. 2, section on the price-motion model.
Example — A nominal 18-week trading cycle may measure 16 weeks in one span and 22 in another; the envelope may no longer be filled, and triangles appear (Chs. 3 and 10).
Note — If MD affects the trading cycle you are using, Hurst recommends temporarily avoiding the issue (Ch. 10). Trailing losses mitigate the damage if you are already in a position.