Decrease means decreasing what is superfluous; increase means increasing what is deficient. 'Three people' refers to the lower three yang and upper three yin.
When three yang walk together, Nine in the Third Place is decreased to increase the top; when three yin walk together, Top Six is decreased to become the third: 'Three people walking, decrease one person.'
The top exchanges yielding for firm, and this is called decrease, merely referring to reducing by one. Although the top and third originally correspond, through the ascending and descending of these two lines, the whole hexagram forms pairs of mutual companionship.
Initial and second are two yang, fourth and fifth are two yin, possessing the same virtue and placed adjacent; the third and top correspond—all are pairs of mutual companionship, so their will is single-minded, and each 'finds his friend.'
Although the third is adjacent to the fourth, they belong to different trigrams and the third corresponds to the top, so they are not companions walking together. Three people decreases one; one person finds a friend.
For beneath heaven, nothing is without duality: the mutual pairing of one and two is the root of endless generation. Three is superfluous and ought to be decreased; this is the great principle of Decrease and Increase.
The Master further exhausted its meaning in the Appended Phrases, saying: 'Heaven and earth blend intimately, and all things transform purely. Male and female unite their essence, and all things are transformed and generated.
The Changes says: Three people walking, decrease one person; one person walking, he finds his friend. This speaks of arriving at oneness.'
'Blending intimately' describes mutual closeness: when the breaths of heaven and earth interact intimately, the transformation of all things is pure. 'Pure' means rich and thick, which is like being concentrated and unified.
When male and female interlace their essence and breath, they transform and generate all things; it is only through refined purity and singleness of focus that generation is possible. One yin and one yang—how could there be a third?
Therefore three must be decreased, speaking of concentrating entirely upon oneness. Between heaven and earth, among the clearest and greatest examples of decrease and increase, none surpasses this.