The Chainfist is a variant of the Power Fist equipped with a small chainsword blade. This powerful but cumbersome (and so slow moving) weapon is often used by Space Marine Terminators, especially by tank hunter specialists or boarding parties because of the great capacity of the weapon to cut through the thickest armour, both vehicles and buildings included.
It is essentially a power glove with a built in chainsword with the cutting edge encased in a glowing energy field. When the weapon impacts, the energy field leaps across to the target, shattering anything it comes across and then the teeth of the blade cut through the remaining flesh and armour with ease.