Dao De Jing chapter 40, in full, reads: 'Reversal is the movement of the Dao; weakness is the function of the Dao. All things under heaven are born of being; being is born of nonbeing.' The Mawangdui manuscripts (tomb sealed 168 BCE) and the Guodian bamboo slips (c. 300 BCE, excavated 1993) both preserve the passage, making it one of the text's oldest attested layers.
Popular culture renders reversal as 'everything balances out' or 'what goes around comes around', a cosmic fairness doctrine. Chapter 40 says nothing about fairness; it describes a mechanism, and mechanisms do not reward anyone.
The paired line is the scandal: weakness is not the Dao's failure mode but its operating method, because only the weak and empty can reverse, yield, and persist. The water, valley, and infant images across the text are this single principle wearing different masks.
If the Dao moves by reversing, what is your current trajectory already turning into?