There are two state for the MG: the charging phase and the discharge phase. You can see which parts are active in each stage based on if the element is black or shaded.

Charging Stage: Essentially, it’s a bunch of capacitors in parallel with resistances in between to slow down the rate of charge. All of the capacitors will charge to the input voltage (ie. 3v)  after a large amount of time. When the last capacitor is charged, then we’re ready to discharge.

Discharge Stage: The entire time, there were spark gaps that stretched across each stage diagonally. These are seen as an open circuit (inf. resistance) unless the voltage across it exceeds a specific value. In that case, the resistance drops to near zero! Lets say for all of the gaps, that threshold is 5v. Once all of the capacitors are charged, the voltage across each threshold is 3v, which is not enough to overcome the spark gaps. Now, if I decide to purposefully short the first stage, then the voltages of the first two capacitors will combine. Then the next spark gap threshold will see 6v causing it to jump over. This chain reaction continues until the last capacitor is met. At this moment all of the capacitor voltages will have added and you will have one big spark at the output.