Power fist
A Power Fist (aka Power Glove) is a large metal gauntlet surrounded by an energy field that disrupts solid matter. It is large and slow in combat, and so the bearer must be willing to take damage before they can strike back.
The benefits of the power fist are that it increases the user's strength, much like Power Armour due to the extra power put in by the electronics designed to allow it to move.
It is most commonly found fitted to Space Marine Tactical Dreadnought Armour. Officers from the Imperial Guard may also take them. It has also been told certain powerfists can and have fired bolts of lightning or focussed electricity out of the fingertips of the gauntlet, sometimes shutting off an enemie's armour/mechanical devices, causing the powerfist to double the punishment it unleashes on its foes.
Commissar Yarrick is most famous for his power claw, which he tore from an Ork Warboss. Other armies also make use of the power fist including Chaos Space Marines, Sisters of Battle, and Eldar. The Orks use a similar device known as a Power Klaw which is a large claw with three bladed fingers and another similar device used on Space Marine Dreadnoughts called a Power Claw has the same effect as a Power Fist.
A variant of this is the Chainfist, which is similar to a power fist but with a miniature chainsaw blade attached.
