I expected that it was a long exposure when I first looked at it, but I don't think it is (solely). If it were a long exposure, his hands and the chain would be blurred where they were moving with the fire.
I'm not a photography wizard, but if I were to do this, I would take one fast shutter photo and one longer shutter photo and combine them. The first would give you the hands/chain/body sharply. The second would give you the fire rings. They you just erase everything but the fire on the second and overlay it on the first. It takes a lot of skill though, as the fire is semi transparent in places (which is why you want to shoot it at exact same place rather than just grabbing the fire from anywhere. It makes it a lot easier.
Now someone with more experience/talent than me could possibly do it more efficiently, but that's how I'd go about it and the end result would look the same
What is a rear curtain sync? (in this case) It's a long exposure photograph with the flash firing as the shutter closes. i think the flash fires at the end of the exposure because of his hands and the direction of rotation of the chain.
[sometimes its not a "long" exposure, but usually is]
Oh, but yeah 100% photoshopped! look at that post crop vignetting and off colours. not to mention the no highlights and super dark blacks. (im a bit colourblind but to me the fire looks green)
imo the photographer should have cropped the right off too