Uniting Yin and Yang is a fragment of a document found in a tomb (in 1973), which dated back to the second century BCE during the Han Dynasty. It turned up in a tomb closed in 168 BCE. It addresses the benefits of intercourse without orgasm. A textual descendant of this item can be found in The Classic of Su Nü (~350 CE).

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The excerpts below come from Art of the Bedchamber by Douglas Wile. Dr. Wile translated various classic Taoist texts.

Excerpts

One arousal without orgasm makes the ears and eyes sharp and bright.
Two and the voice is clear.
Three and the skin is radiant.
Four and the back and flanks are strong.
Five and the buttocks and thighs become muscular.
Six and the “water course” flows.
Seven and one becomes sturdy and strong.
Eight and the pores are lustrous.
Nine and one achieves spiritual illumination.
Ten and the body endures.

This next, similar, fragment was part of another document found at the same time, entitled Discourse On the Highest Tao Under Heaven.

One arousal [without orgasm] and the ears and eyes are sharp and bright.
Two arousals and the voice becomes clear.
Three arousals and the skin becomes radiant.
Four arousals and the backbone becomes strong.
Five arousals and the buttocks become muscular.
Six arousals and the “water course” flows.
Seven arousals and one becomes stout and strong.
Eight arousals and the will is magnified and expanded.
Nine arousals and one follows the glory of heaven.
Ten arousals and one manifests spiritual illumination.