Peaberry bias section

[W4MMP]

Hi,
Would someone explain how the bias section of the final amplifier works in detail? I understand the basic concepts of class A/B amplifiers, gain, trans conductance, feedback,and such. But I have studied that circuit and just cant quite get my head around it. Questions like: What is the purpose of D2?

Just curious, no other reason.

[W4MMP]

Hey folks,
Surely, someone knows how it works