Notice the flash is a bare-bulb type (A Quantum Q-flash with the reflector removed),and an assistant is running around the scene making flash pop after flash pop....how many pops were there? 150 or more? Note that the flash-holder's hands do not show. He was probably wearing all black,including gloves.
Dean Collins did a talk about this technique 25 years ago. He would have his black-clad assistants, aka "ninjas" moving through difficult to light interiors and adding light with flash units while he did long exposures on large format film using daylight-balanced film. They would hide the light in front of their bellies, hunching up their shoulders, to prevent the flash from showing them in silhouette. This shot is a bit different though.
You have to make sure that your "Ninja" does not accidentally illuminate him or her self while making said 150-200 flash pops, hence the need for black clothes and being on the move. It's a neat picture! Thanks for showing it to us.