廣弘明集 9.130

9.130

臣笑曰。臣年二十之時。好道術就觀學。先教臣黃書合氣三五七九男女交接之道。四目兩舌正對。行道在於丹田。有行者度厄延年。教夫易婦惟色為初。父兄立前不知羞恥。自稱中氣真術。今道士常行此法。以之求道。有所未詳。

Your subject [Zhen] Luan smiles and says: When I was twenty years of age, I was fond of Daoist arts and went to an abbey to study. They first taught me the Yellow Book method of harmonizing qi through three, five, seven, and nine, the way of copulation between men and women. Four eyes and two tongues face directly; practicing the Way lies in the elixir field. Those who practice it deliver themselves from adversity and prolong their years. Teaching husbands to swap wives, it takes lust as primary; fathers and elder brothers stand before them without any sense of shame, calling it the ‘True Art of Central Qi’. Nowadays Daoist priests routinely practice this method to seek the Way, which is quite unfathomable.