Trigger analysis for a Half Bridge Self oscillator
The DIAC is a negative resistance clamp that form a relaxation oscillator to start from R 470k pull-up and 22nf. It is like a 2 pin SCR or unijunction and  goes into the base then the collector diode actually discharged the cap further once switched on briefly.

This creates a sawtooth  drive to get the oscillator started and pulse dump into the base .


  
  1) Assume input is 300Vdcand start current in 470k is
  Ic=0.64 mA as 22nF charges up
  C =22n and
  dV is 32V for DIAC V_BO    
  
  Min Typ Max
    28  32  36V
  dt the time to trigger so from Ic=CdV/dt
  dt= CdV/Ic=22n*32V/0.64m= 1.1ns assuming 1/2 cycle f= 1.8kHz
  2) when DIAC Vbo is reached voltage drops to 0.5V and current > 10mA with low negative resistance like an SCR latch.   
  
  
  It will latch on until current drops below Ibo 100uA, which is less than above so voltage must be dropped further using Diode and transistor saturated by DIAC pulse.
  
  3) now the coil T1 10T is energized by lower NPN collector in series with T2 primary 88T  the other side of T2 is 300Vdc/2 with filter caps, lets assume now as an AC ground ( capacitance voltaqe divider for AC current.)
  4) This pulse duration depends on  T=RC decay time of 22nF and DIAC Rs
  5) the T1 magnetics  forms a blocking oscillator similar to a Joule Thief with T2 getting the AC square wave voltage and triangular current to step up current and step down voltage.
  6) as lower NPN-C goes low, upper E also and dot in 10T goes low as base in upper NPN coil dot goes high. as V=LdI/dt Vbe rises and then triggers while shutting off opposite dot direction on lower NPN.