Voltage of 400kV would allow to have a spark almost half a meter long at standard atmospheric pressure. Here we have 10 to 20cm what means 10 to 20kV and no more.
Basically, it works but the description is missleading.
Unfortunately, It seems the voltage here is like the resolution of chinese cameras or power of laser leds ... in reality it is much lower than in the description.

The kind of spark with bang and flash looks like the effects wich are seen when professional tasers are used.
The chargetime of the HV capacitor (there must be one inside) influence the frequency of sparking and als the amount of joules one discharge delivers.
When used for physics, keep the inputcurrent low and the sparkgap small. Also an array of at least 20 resistors 22KOhm come in handy.
These actions will make this powerfull generator more civilised.
Although I have experienced with many HV systems this relatively small unit will shock you very hard when you are foulish enough to try how hard.
This generatir is not suitable to replace a Rhumkorff spark inductor, van de Graaf- or a Wimshurst generator.
It can however be used in the primary of a Teslatransformer, in wich case 400KV can be reached (use extra capacitors+ resistors to reduce chargecurrent and sparkgap max 10mm.)
runningtime a few seconds.
I think it can be used (in a modified setup) to make an electric fence.
For igniting a gasflame it has an overkill, the 7KV unit Banggood sells is more appropriate.
The max voltage exeeds 20KKV but 400 KV or more is an exaggeration.
Because these kind of generators can damage themselve easily,
you can purchase 3 or 5 at one time, so you have a spare!
(Murphy's law, you know)