The website explained that the POCO had been having a lot of difficulty with this switch clearing the line, so they had set up this video test to pinpoint the problem. Look closely at the front left-hand horizontal insulator (actually a SF6 interrupter bottle) at the very start of the video. You'll see a flashover arc because it's companion on the right failed to open properly, allowing the flashover. That part of the arc goes out as the bottle contacts reclose.
The proper cyle for this switch is: open commanded-linkage starts to open-interrupter bottles open[internal contacts]-main contact begins to swing open-interrupter bottle contacts reclose-main contact finishes opening.
The main arc went out because the POCO had an upstream Oil Circuit Breaker open. (Another source reported that the OCB was also badly damaged by this incident.)
Luckily there was no wind at all or that arc could have easily gone phase-to-phase. My POCO friend explained that if that had happened, it was highly likely none of those workers in the video would have escaped unhurt.