Hi Paul, we are getting off topic but in short:
The circuitry around IC4 is an inverting comparator with hysteresis (see https://www.ti.com/lit/an/snoa997a/snoa997a.pdf?ts=1609852900466&ref_url=https%253A%252F%252Fwww.google.com%252F).
Via IC5 it starts passing the signal to the MOSFET drivers (IC6, IC7) as soon a the amplitude at the input (rectified and amplified by IC3) exceeds a "high value" (eg 300mV) and keeps it passing until the amplitude drops below a "low value" (eg 100mV). That way there are not instabilities during a slow rising of falling signal.