If one side destroys the enemy it wins automatically, even if it sustained more casualties. They may consolidate as normal (see page 10).
In addition, the destruction of the unit will unnerve nearby friends if they are not already engaged in close combat. Immediately take a Morale check for every unengaged friendly unit within 6" of the victorious unit before it consolidates. If the check is failed, the unit in question will fall back as described below.
Diagram 8. Wipe Out: Unit 1 is within 6" of the enemy which wiped out its opponents and must check Morale. Unit 2 is more than 6" away and does not check Morale. Unit 3 is locked in close combat and so does not have to test.
Designer's note: This rule is borrowed from Warhammer, which quite sensibly supposes that troops seeing their fellows wiped out to a man will start giving serious thought to hoofing it before they're next! It means that tough assault units can break an enemy gun line, throwing the foe into confusion as they try to pull back. This helps balance off some of the risks intrinsic in braving enemy fire to assault in the first place, and the vulnerability of assaulters to get blasted with supporting fire the moment they emerge from a close combat.