If you follow the dev diaries of FO3, they're pretty open to the fact that all of the weapons are their own "neat" and "cool" ideas. Practically none of the new incarnations of the original weapons were intended to be modern explanations of the unexplained originals, but rather Bethesda's own reimaginings of them. As far as they were concerned, the Rail Gun and Power Fist were the same thing; cool. And naturally, like most of the game, FONV's Power Fist just carried over from FO3's. The modern Power Fist really shouldn't be counted as an explanation as to how the original Power Fists worked, because they were deliberately different interpretations of the "same" weapon.
I was under the assumption we were trying to figure out what the original Power Fists did, because in those same dev diaries, Bethesda was pretty clear with how theirs worked. Hydraulic fist. But the original Power Fists looked nothing like that. They appeared to be little more than fully-metal (badass looking) gauntlets, but they were powered by Small Energy Cells. Hence my understanding of it being a really BIG "brass knuckle" with electrocuting properties.