There are confusing and contradicting teachings regarding the principle of Ling and Zheng 零正. Usually the period star according to San Yuan Jiu Yun 三元九運 is placed at the center of a nine square grid and allowed to "fly forward" to complete the Heaven chart. The location of the number 5 in the chart is considered the Ling Shen direction. The opposite direction is the Zheng Shen direction. However, Tan Yang Wu taught something different.
He said that if the number is odd, then it flies forward as odd is yang. If the number is even, then it flies backward as even is yin. This is according to the Yang forward Yin backward 陽順陰逆 principle. For example, in Period 8, this is the Heaven chart:
9 4 2
1 8 6
5 3 7
5 in the NE is Ling Shen and there should be the place for water. The opposite direction SW is the Zheng Shen direction and there should be mountain.
This is one of the interesting advocations. Stay tuned for more.