Precision diode is use for half wave rectifier and peak detector
When Input is positive then there is positive voltage at Diode which switches the diode on when Input is negative then there is negative voltage at diode which is open circuit Diode.
With a little modification, the basic precision rectifier can be use for detecting signal level peak. In the following circuit, a capacitor retain the peak voltage level of the signal and switch is used for resetting the detected level.