It means that the God of Grain goes in and out of the door, and goes out and in here, breathing finely and deeply, from the mouth and nose to the fetal breath.
This sentence comes from "The Supreme Jade Emperor's Heart Seal Miao Sutra", which is a method that praises the cultivation of inner elixirs. The author is unknown. It dates from the Tang and Song Dynasties and was written by Min Zhiting on October 7, 2009. annotation.
"Tao Te Ching" says: "The immortality of the Grain God is called the mysterious female, and the gate of the mysterious female is called the root of heaven and earth." The God of Valley is the aura in Xugu, the Yuanshen of Tiangu. Xuanfei is one yin and one yang. The spirit in the heart is Xuan, the qi in the kidney is female, and the emptiness and Qi produced by the union of spirit and qi is the God of Grains. If we talk about breathing, we can also say that exhalation is Xuan, inhalation is female, and the purity cultivated by breathing is the god of grain.
Extended information
"The Jade Emperor's Heart Seal Miao Jing" is a Taoist classic. Lu Zu said, "Only cultivating one's life but not one's nature is the first disease of cultivation; if one only cultivates one's ancestral nature without cultivating elixirs, it will be difficult for the Yin spirit to become a saint for thousands of eons." This sutra is the method of practicing life skills.
The Heart Seal Sutra explains the mystery of the Tao and reveals the foundation of life. The true path is the ladder of salvation. Scholars must be able to formulate their principles and express their words. They must use the spirit to understand and transform, and they must understand the mysteries to understand the wonderful things. Knowing the heart is the sect of the whole body, and there is no loss of self-cultivation.
Don’t block it, don’t close it, the four gates are bright, the sky is calm, and the empty room is white, that is, the heart is the seal, that is, the seal is the heart, the heart seal is blended, the upper and lower holes are clear, like the moon appearing on the river, like the stars covering the sea , the vacuum is silent, one nature is transcendent, and one does not know which is the heart and which is the seal. As for forgetting all the seals of the heart, and both God and Tao return to their innocence, then the wonder of the seal of the heart can be obtained by oneself.
Baidu Encyclopedia—The Jade Emperor’s Heart Seal and the Wonderful Sutra