Where a side has won and their enemies fall back, the victors can choose to either advance or consolidate their position. If they won on a tie-breaker they may only consolidate. They may make this decision after the enemy has fallen back (ie you can see how far they run!).
Diagram: Advancing. The Space Marines win an assault and the Dark Eldar fall back. The Space Marines advance 10", annihilating the falling back Dark Eldar, who have only managed to move 5".