Consider this discharge Circuit. What does the input do?

Electrical schematic of a high-voltage discharge circuit. At left, INPUT passes through resistor R1 to pin 1 of an optocoupler LED; pin 2 returns to GND. The optocoupler's transistor side has pin 5 connected to a control node and pin 4 to the HV− rail. That node connects upward through R2 (marked 1M) to HV+, downward through a Z-DIODE to HV−, and horizontally to the gate of an NFET. The NFET is in series with R3 (labelled RDischarge) between HV+ and HV−; the resistor is above the transistor.