Meph is right, but the winner has a choice about whether to take the Withdrawal. The winners can choose to sit in place and not suffer the consequences for ending a withdrawal within 15cm of the enemy.
Quote:
1.12.9 Winner Consolidates
After the loser has withdrawn, the winning formation receives
a number of Blast markers equal to the number of units that
were killed by the enemy. Note that it is possible for a
formation to win an assault and then become broken by the
casualties that they suffered! In this case, the winning
formation is broken, but does not have to make a withdrawal
even if other enemy units are within 15cm (see 1.13.3).