I'm guessing from the way the rotational force changed on the hand-cranked dynamo that the two thyristors are being fired in sequence. The first one to attempt to fire the connected charges, and the second to shunt the capacitor via a resistor to turn both thyristors off and make the output safe on each firing cycle.
This device is very pleasing in so many ways. It's a proper blasting unit and feels chunky and functional. It can handle between 1 and 100 detonators wired in series, and uses just under 1000V to ensure a high enough current flows to trigger them regardless of circuit length or resistance of connections. one nice feature is that it has a built in resistance meter to give an indication of the circuit integrity before actual firing.

Here's the manufacturers website, but note that their products are not really aimed at casual users. Their main markets are quarries, mines, demolition and military applications.
https://arbra.co.uk/

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