How does a class G/H amplifier work?
This extends the power saving efficiency technique of Class AB beyond just switching at the zero crossing voltage to switching also at +/- 35V-Vf.

In  Class AB mode  Q2/4 alternate as power drivers to the load at the crossover of 0V bias.  Let's assume Vf=0.5V for simplicity , so Q2-C=34.5V in Class AB mode.   When the signal rises to start conduction  of the  3.3V Zener on the +ve side, the Zener cathode will be 3x 0.65V drops above the collector Q2-C or about 2V+ 34.5V.

So the Emitter Follower Darlingtons amplify the current and the Class GH switches in like AB except instead of just zero-crossing bias controlled by Q9 for split power for Q2/4, the Zener and Schottky diode near 35V crossing  linear switches.  The idle current allows the soft switching for transferring power from 35V supply to the 70V supply in order to reduce the power loss across Q2/4 so that they begin to saturate when  when Q6/8 begin to conduct from +/-70V.