Observing the natural Dao, there is nothing to observe. Don’t use the eyes to look, use the heart.
The heart is deep and subtle and cannot be seen.
If you can see the natural mind, only then can you illuminate the deep subtlety. This is why it is called “yin.”
Grasping the movement of nature there is nothing to grasp. Do not grasp with the hand, grasp its nature. Its nature changes constantly and has nothing to restrain it. If one can agree with nature, this is called “fu.”(sealing).
Illuminate with the mind, agree with its nature and the yin seal is already complete.
Li Quan believed that yin was the dark, using these words to lead the text, how could anyone be completely unclear? – zhang guo commentary of the first passage of yin fu jing