Can someone explain the SSL mixbus Ratio switch circuit?

Hi All,
I have been studying the SSL Mixbus compressor sidechain ratio switch circuitry, and I just haven't been able to figure out the operation of the resistor network and diode used for the 1:2 ratio setting.

Can somebody here explain to me what the network, especially the diode, is doing in the circuit?

It's SW1 on Gyraf's GSSL schematic (below), right side, halfway down:



And does the 1:2 ratio diode and resistor network do anything when the ratio switch is set to 1:4 or 1:10?

Thanks,
mr coffee

My Horowitz And Hill, Art Of Electronics, in Russian, have this picture:



Kinda rough explanation: