OK so in FO3 the power fist works as sort of a hand mounted jack-hammer how did the orgional Power fist increase punch-strength?

Energy cells powering a kinetic energy storage device to increase knockback.

It also run on small energy cells unlike F3 variant that seems to be more like the Fallout Tactics Impact Gloves.

thank you, that's what I wanted to know, so it's similar to the super sledge with the kinetic energy storage device to increase knockback