The KMD Multiwrench has three primary functions: repairing vehicles and ship components, repairing armor to other soldiers, and reviving downed players.
You can't revive anyone, however. Only allies who are bleeding out (signified by a symbol resembling a heart monitor flashing over their head) can be revived. If you're doing it right, their armor's cracks will repair themselves and they'll get back up on their feet.
Unlike most sidearms, the Multiwrench can't be equipped in the Primary or Secondary slots, only in the Sidearm slot.