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. 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 last in close combat regardless of relative Initiative and whether or not they are defending cover or using frag grenades to assault into cover, etc. If models on both sides are armed with power fists they strike simultaneously.