The circuit is based on operational amplifier IC1 used in the noninverting amplifier mode. Rl and 2 set the gain at about 500. RV1 (sensitivity) biases the noninverting input to the negative supply. Ql provides the relatively high trigger current required by the triac. When a signal is received by the microphone, the signals are amplified (by IC1). The triac is triggered and a low resistance appears across its Al and A2 terminals which are connected via the fiashlead to the strobe.
The circuit operates almost instantly, giving very little delay between the commencement of the sound and the flashgun being triggered.