Clock generator with two inverters - how does it work?
What is that R3/D1 line for ?


Revised to correct a typo!

D1 is the feedback - when IC2A output goes low, it discharges C2 to about 0.6V. This sets IC1E o/p high, IC1D o/p low and IC1 o/p goes high. D1 is now reverse biased and C2 charges thru SW1 and R4 (100k).

Eventually C2 has enough voltage on it to make IC1E o/p low, IC1D o/p high and IC1A o/p low thus we are back to the beginning of my answer.