The coil is connected on one side to positive 12V. The other side is connected to a MOSFET which is connected to GND through a second MOSFET. So by modulating the signal applied to the MOSFET gate, we modulate when the circuit is closed and power applied to the flyback coil. Music signal is also connected to the base of a two BJT transistors that create an inverter. Then we connect the output to some other BJT transistors in cascade and with that we amplify that signal, because the music voltage from a smartphone if very, very, low. That output is used to apply pulses at the LC tank and make it resonate. On top of the resonance frequency which is given by the used components such as the 330nF capacitor and the inductance of the primary coil from the flyback, on top of that, we insert the music signal as well and that’s how we get the music on top of the plasma arcs. The second MOSFET is connected to GND and is activated with the sliding switch, and that turns the oscillation on and off. Is a pretty easy circuit.