hi
When the voltage across NE2 reaches its breakdown voltage, the current tiggers the SCR on, this discharges the capacitor thru the high voltage trigger transformer primary winding, causing a 'flash'
Thanks Eric, wasn't sure how the neon developed the build up voltage in this circuit, other similar circuits seem to feed the neon voltage through a 1 meg resistor into a charge capacitor and then feed the other side of the neon into the SCR gate like this:-
My numbers might off by a few... But a neon lights at around 70 volts, which also where the gas starts to conduct. The xenon needs 250-300 volts to light, plus in needs a high voltage arc to ionize a path (3 kV or so). The resistor makes sure the neon doesn't conduct, until the capcitor is charged enough to light the xenon.