Force weapons are potent psychic weapons that can only be used by a trained psyker such as a Librarian. They are treated as a power weapon, but can unleash a psychic attack that can kill an opponent outright. Roll to hit, to wound and to save as normal. Then, as long as at least one wound has been inflicted, make a Psychic test for the psyker against one opponent wounded by the weapon. The normal rules for using psychic powers apply, and you can not use another psychic ability in the same turn. If the test is passed then the opponent is slain outright, no matter how many wounds it has (but count the actual amount inflicted for determining which side won the assault).
Note that a force weapon has no special effect against targets that don't have wounds, eg Dreadnoughts, vehicles, etc. Also note that you only take one Psychic test no matter how many wounds were inflicted.