Supporting Attacks
Models in the second rank aren't idle while their comrades fight -- they muster forward and strike blows of their own, called supporting attacks. A model can make a supporting attack if it's directly behind a friendly model that's itself fighting an enemy in base contact. Supporting attacks can never be made to the side or rear, and never by a model that's itself already in base contact with an enemy -- it must fight its own more immediate foe instead.
A model making a supporting attack is more hemmed in than one fighting face to face, so it can only ever make a single Attack, regardless of its Attacks characteristic or any bonus Attacks it would otherwise get. Supporting attacks are directed at whatever the front rank model being fought "through" is itself fighting -- if that front rank model is in contact with two or more enemies of different profiles, the attacking player chooses which to direct the supporting attack against, before dice are rolled.