Any living or non-living thing on Earth changes over time, not only due to external forces but also because of internal development. The eight-character destiny chart formed based on the birth time is a fixed state. Its five elements' interactions change with the flow of the years, but there should also be an internal mechanism of change.
Accordingly, the theory of eight-character fortune-telling introduces the concept of a "Major Period", which is a dynamic parameter. "Yun" means movement, and it is called a "Major Period" to distinguish it from a "Minor Period". The stem and branch of the Major Period are derived from the month pillar of the eight-character destiny chart. If we consider the four pillars plus the Major Period as a complete destiny chart, then the Major Period becomes the "moving" pillar of the chart. This conceptual "destiny chart" truly reflects the principle of continuous internal change in any thing, showing the curve of life changes. There are those who talk about the "five-pillar" theory, which refers to this idea.
The developmental stages of the Major Period. We say that the Major Period is like the moving pillar in the destiny chart, but it doesn't change with the flow of the years. Instead, it changes in stages every ten years. During each ten-year cycle, the Five Elements energy of the Major Period's stem and branch remains relatively stable. The Zui Xing Tang school of fortune-telling does not support the idea that the stem governs the first five years and the branch governs the next five years. From this, the theory that the stem governs the first half of the year and the branch governs the second half is also not accepted by Zui Xing Tang.

Every year we have experienced or will experience flows like water, each with its own stem and branch Five Elements energy field, determining our luck for that year. In fortune-telling, this is called a "Flow Year". Interpreting the luck of each year is called "interpreting the Flow Year".
Each Flow Year's stem and branch controls the Five Elements energy field of that year, and in a way, has absolute authority. Therefore, ancient fortune-tellers regarded the Flow Year as an inviolable "Tai Sui" (the God of the Year), believing that Tai Sui (the Flow Year's stem and branch) has power above the destiny chart and the Major Period's stem and branch. Although this view has some merit, it should not be exaggerated. Although the energy field of the Tai Sui stem and branch fills the universe, when it affects each person's destiny, it is similar to the energy field of the destiny chart's stems and branches. For example, although Earth's gravitational pull is huge, why don't we get pulled into the center of the Earth? Because a certain mass can only withstand a certain amount of force. The relationship between Tai Sui and the destiny chart is the same.
Therefore, the Tai Sui stem and branch participate in the generation and control within the destiny chart, and are also affected by the destiny chart's stems and branches. However, the effect of the Tai Sui stem and branch is dynamic, occasional, and superficial, but it can trigger static, inevitable, and deep-seated effects within the destiny chart.
In the past, there has been a saying, "The Major Period emphasizes the branch, and the Flow Year emphasizes the stem." From the previous discussion on the relationships of the stems and branches, we found that the energy field of the branch is stronger than that of the stem. Therefore, this also applies to the Major Period and the Flow Year. Generally speaking, the energy field of the branch is more important. However, the stem is superficial, and the events of the Flow Year often reflect the image of the stem of that year, giving the impression that the stem is more important. In fact, what happens in a given year is the result of the interaction among the six stems and branches of the Flow Year, Major Period, and Destiny Chart. The stem of the Flow Year does not dominate the situation.