Consider this discharge Circuit. What does the input do?

Electrical discharge circuit schematic with high-voltage rails labelled HV+ (top/right) and HV− (bottom/right). The INPUT at the left passes through resistor R1 to pin 1 of an optocoupler LED, with the LED return pin 2 connected to GND. On the isolated output side, optocoupler pins 5 and 4 connect to a control node and HV−, respectively. The control node connects through R2, labelled 1M, up to HV+, through a Z-DIODE down to HV−, and horizontally to the gate of an NFET. The NFET is in the lower portion of a discharge branch, with resistor R3 labelled RDischarge between HV+ and the NFET drain and the source connected to HV−. The diagram includes no explicit operating-state arrows or voltage measurements.