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 second principle of the Hurst cyclic model: each periodic component varies slowly over time in both amplitude (magnitude) and duration (period).
It is not a model bug — it is structural to price.
Ejemplo — A nominal 18-week trading cycle may measure 16 wk in one span and 22 in another; the envelope may no longer fill → triangles appear (Ch. 3, 10).
Note — If MD hits the **trading cycle** you are using, Hurst recommends **temporarily avoiding** the issue (Ch. 10). Trailing loss mitigates damage if you are already in.