When you first clamp your hands, mic detects the sound and triggers the 555 timer via the operational amplifier. The timer stretches the signal and applies it to the clock input of the flip-flop.
Over on YouTube [Andrew Neal] has a Function Generator Build for Beginners. As beginner videos go this one is fairly comprehensive. [Andrew] shows us how to build a square-wave generator on a ...
The function of the 555 timer is mainly determined by two comparators. The output voltage of the them controls the state of the RS flip-flop and the discharge tube. It also contains 3 working states.