SanYuan 三元 is one of the system of keeping track of an event occur in the space-time continuum 時空. The term 元 implied space-time continuum 時空. The notion of 三 implied upper, middle and lower - the trilogy of early, later and middle Heavens (past, current and future events). Each of the 元 has its own layers. In simple term, 三元 is a space craft travels in the space time continuum.
As SanYuan developed on its very own in conjunction with the existence of the early calendric systems, it has borrowed many existing knowledge notably the idea of Li, Qi and Xiang 理气象. Li 理 specify its method, Qi 氣 adopting its five elemental 五行 units as 甲子 and Xiang 象 encoding of both into a system of Gua 卦 binary. SanYuan Method works in a rotation or spirals. A continuum spiraling space-time has no gap in between that is why it is continuum.
In reality, SanYuan operates beyond our 3D existence. However, due to the limited crude technology available to our forefathers, space 空 has been compressed into 2D space fondly known today as the 9 palaces grid 九宮. This became the hall mark of SanYuan. When Time 時 is induced within the 9 palaces, it takes the form as JiaZi 甲子. Technically space-time in SanYuan is measured via the JiaZi running about within the 9 palaces grid.
Within JiaZi, let’s look at the stem 干 itself. There are 10 stems but there are only 9 palaces. So, at any one time, only 9 stems will be shown array within the 9 palaces. If you need to count from the first stem, 甲 all the way to 癸, it is always the Jia 甲 that will be gone missing. This is how the term DunJia 遁甲 came about.
DunJia always work as a rotation within the SanYuan enclosure of 360 days a year. The remaining days are calibrated via a method called ZhiRun Fa 直闰法. However, it is not always easy for common people to keep up with the intercallaration of this method. Therefore, they improvised another method called ChaiBu Fa 拆补法, meaning dismantle and patch back again. However, this method is a little arbitrarily and not truly the pedigree of DunJia.
DunJia 遁甲 is further developed into its own method notably, the art of war 兵法. By resembling Jia 甲 as the King 君, YiBingDing 乙丙丁 as the 3 nobles and the rest of the 6 stems as the 6 crescents or warlords, it has manifested itself into an art governing human affairs. That is what it is famously known today by literatures of the 3 Kingdoms and so forth. So hiding the Jia or 遁甲 becomes, hiding the King.
However, it is always not necessary true. DunJia is also a system enable one to peep into possible future resembling Quantum Mechanics where the anthropic effects do not necessary bring about a fix definite outcome, 1 plus 1 is never equal 2. This is later commercially termed forecasting. In fact, many of the forecasting system we use such as 六爻, 八字, 玄空 and so forth are based on DunJia. Strangely the work of BoWen, DiTianSui 滴天髓 Drips of Heavenly Marrow laid the foundation of DunJia.
With forecasting, it is then applied to FengShui forecasting. It is never meant to be a FengShui method. It is deemed to have been a FengShui method by practitioners who had never been trained in true YangGong FengShui or DiLi 地理. So, technically it should not have been use as a FengShui technique but a FengShui forecasting instead. It gives you a glimpse of where Qi is coming from, going to and its apparent Sha 煞. You still need to adopt 收山出煞 Receiving Yin, Protruding Sha to ride Qi. Ironically, BaZhai 八宅, ZiBai 紫白 and XuanKong 玄空 are all the offsprings of DunJia 遁甲. So, how on earth are these techniques used to 收山出煞 Receiving Yin, Protruding Sha to ride Qi? Strange isn't it?
Even more strange when there are people who says BaZhai 八宅 using the 甲子 JiaZi is never a true method because in the olden days, it simply cannot be that complicated. How on earth could an offspring do not contain the DNA of its parent? How do you think all the Gua numbers 命卦 comes about if it is not from DunJia? If you cannot discern these, please go back to the fundamentals.
With forecasting also, it is applied to Fate calculation 命理. It cannot replace BaZi but it could point out a way to go against destiny, because with forecasting, the end result is not fixed. Some use BaZi as the diagnosis and DunJia as a prescription.
Collectively, art of war cum destiny cum forecasting are sexily termed strategy execution coupled with QiMen, it is now known as QiMenDunJia 奇门遁甲. Aptly, SanYuan QiMenDunJia 三元奇门遁甲. It adds many layers to QiMen to make it more complex now having the dimensions of deities, stars, doors, heaven and earth Qi, PurpleWhite, 9 palaces and Gua.
Not sufficient? Wait till you encounter this species called XianTian SanYuan XuanKong DaGua QiMen DunJia 先天三元奇門遁甲玄空大卦. What a mouthful of jargons. Strangely, this is not yet the material for XieZiFa 些子法.