The trick is multiple flashes - it's about the only way of achieving such clarity and such an interesting composition.
The downside to this is that there's a lot of equipment required.
It's certainly possible to get people to stand still for 1/60 second, but 30 seconds is a lot more challenging. And again, conversely, it's easy to carry around a single high-power LED torch, but a lot more hassle to carry four flashes.
At this juncture, I think I'll keep quiet about the dozen or so Metz flashes that I've sold in the last 18 months, suffice to comment that I suspect the ones used in this case probably didn't have the bulky external battery packs :confused:
'Open flash' certainly is going to deliver better results, it's just a shame that there's a lot more hassle involved.
Hello again darkness, my old friend...