Qi Men Dun Jia, a highly sophisticated field in the chinese metaphysics, is sometimes also known as the Study of Heavenly Stems (天干学).
Why is this so is because the 4 very important stems that Qi Men practitioners focus on are namely Jia wood, Yi wood, Bing fire and Ding fire. Oh, and one more, that is, Geng metal – the very much hated enemy of Jia.
Qi Men Dun Jia was orginally used as a war strategy and the Heavenly Stems were in a way arrange to develop formations to protect and conceal the leader of the army battalion from the enemy. Analogously, Jia wood is the leader or highest commander that we want to protect from Geng metal. Bing and Ding fire are recognised as 2 of the most effective generals to counter and control Geng based on the 5-element relationship.
What about Yi wood, which is the sister of Jia? How is she going to handle the ferocious Geng which is the fiercest among all the 7 Killings? Yi wood is being chosen here to protect the Jia wood due to the fact that Geng is afraid of Yi.
According to the Ba Zi Classic Di Tian Sui, it says: 庚金带煞，刚健为最。得水而清，得火而锐。土润则生，土干则脆。能赢甲兄，输于乙妹
Put it this way, Geng is henpecked and knowing that, we use Yi to ”decapicitate” Geng.
In gist, Yi, Bing and Ding are known as the San Qi or 3 Mystical. Combining with an auspicious Men or Door, it provides the right moment and direction to proceed to conquer your opponent. It is a very 3-dimensional approach in ushering auspiciousness for the endeavour that you are going to embark on such as job interview, negotiation and as crazy as using it to enter casino and buy 4-D.