Lembeh Gulen Shootout - Charlton's filefish photo - how?!

I keep returning to this photo and still haven't figured out how it was made. A buddy swimming parallel on the other side with a backlight? A crazy long strobe arm with a snoot hanging over the fish? A serendipitous burst of concentrated bioluminescence? Someone (maybe Richard Charlton himself) please enlighten me!

I would suggest it was a fellow diver with a light(torch)

That does seem most likely, of course. If so the torch must have been very close judging from the backscatter, and the fish moving very slowly, but then it doesn't seem consistent with the illumination on the bottom. Or I am just over-thinking it. Whatever the case it is an impressive image.

Yes, it was a torch from behind

Looks too powerful to be a dive light or torch in British lingo. For me, the noun version of torch is something with flames leaping from it! :->> A slaved strobe, maybe with a lensed snoot like the LSD - note the sharp-edged spot of light on the bottom. Maybe a night dive?