A power fist or claw is a powered, armoured gauntlet surrounded by an energy field that disrupts solid matter. It is an awesome weapon most commonly seen fitted to Space Marine Terminator armour.
A power fist or claw doubles the user's Strength up to a maximum of 10 and ignores Armour Saves. Only the user's basic Strength is doubled - any additional bonus for special abilities is added after Strength has been doubled.
The only drawback is that a power fist is slow and cumbersome to use, so a model armed with a power fist or claw always strikes at Initiative 1 no matter what its bearer's Initiative is (including any bonuses for special rules, cover, grenades or wargear - power fists always strike at Initiative 1 no matter what!).