The way that this circuit works is as follows. The AC line voltage is rectified by D1 and D2 which connects to a voltage doubler circuit made up of the two 22uf capacitors. The Flash Freq. Pot and the 10uf capacitor charge up which triggers the Diac and causes the triac to turn on. This allows the trigger transformer T1 to send a very High Voltage to the flash tube and lighting it. The flash timing is adjusted by the flash freq. pot.