Understanding how the simple precision rectifier works
When the diode becomes forward biased it effectively becomes a short and can be ignored.
Because of &quot;OpAmp&quot; action the OpAmp tries to drive the two terminals to the same level. When Vin &lt; 0 the diode forward biases. The diode will have some finite amount of voltage drop but the OpAmp will null it out because the inverting terminal is sensing Vout dirrectly.
R1 is just there to drag Vout back to GND as Vin comes back to zero from the negative halve cycle.