The power fist is a mechanical glove mounted with a hydraulic jack. When punching, the jack pushes forward, smashing into the enemy. Sometimes, enough power is transferred to the target to knock them down.
If you look closely, you can see notches on the fist that add up to 33.
As of the Old World Blues add-on, the power fist attacks much more slowly, even with a maxed out Unarmed skill and high Strength. Even with the Slayer perk, it punches rather the same speed as it did in Fallout 3. The slower attack speed is not present in the other variants like the zap glove and ballistic fist, however.
Bugs
PC The power fist does not use a higher resolution texture when used by the player, despite one being available in the game's resources.
PC The power fist uses the default Pip-Boy image from Fallout 3, despite there being an updated version present in the game's resources.
Xbox 360 Sometimes when appearing in third-person player, the power fist will appear above the hand.