D1 & D2 simulate the optoisolator diode input of the bias circuit. R2 simulates the relay coil resistance. This was done so I could use "CircuitMaker" to simulate basic operation. The base of Q2 is not grounded as shown. It is that way to facilitate the transmit view of the circuit using CircuitMaker.
So, when the relay turns on, part of the current flows through the diode input of the opto which removes the cut-off bias from the 4-400A grid and applies the "idle" bias.
The basics of this circuit also came from Richard's WEB page.