A Saju chart is fixed at birth. 대운 — usually translated as luck cycles or luck pillars — is how the system describes the chart moving through time.
Each 대운 is a ten-year period represented by its own stem and branch. Those characters enter your chart and change the balance for a decade: an element that was missing may arrive, an element that was already heavy may double.
Direction. The sequence runs forward or backward through the sixty-cycle, determined by the polarity of your year stem together with your sex at birth. Yang year with male, or yin year with female, runs forward; the other two combinations run backward. This is the only reason a Saju form asks the question at all — it is not used for interpretation, and when it is left unanswered the luck cycles are omitted from the reading rather than guessed at.
Starting age. Your first 대운 does not begin at birth. Its start is calculated from the distance between your birth moment and the adjacent sectional term (절기) — the twelve points that begin each Saju month — converted at a traditional ratio of roughly three days to one year. This is why one person's cycles turn at 4, 14, 24 and another's at 9, 19, 29.
How to read it. A 대운 is not an event schedule. It changes what the chart has too much or too little of, which shifts what tends to be easy and what tends to cost effort.
A concrete example of the logic: if your chart runs light on Fire and a Fire-heavy decade arrives, the thing you previously had to seek from other people is temporarily available. What people usually notice is not luck — it is that a familiar difficulty stops being difficult.
There are also annual cycles (세운), which work the same way over one year rather than ten.
Two honest limits. The cycles describe conditions, not outcomes. And the ten-year boundaries are not switches — the transition is gradual, and the pillar's two characters are often read as weighted across the decade rather than acting evenly.
For entertainment and self-reflection only. Not advice.