The side that loses a combat must take a test to determine whether it stands and fights or turns tail and runs away. This is called a Break test. You need to take a separate Break test for every unit involved in the combat.
Depending on which units pass and which fail their test, some may break and flee whilst others stand their ground. Troops which are better led, braver, and more professional are more likely to stand firm, whilst wild, temperamental troops are far more likely to run for it.
Take the test as follows. Firstly, nominate which unit you are testing for. Roll 2D6 and add the scores together. Add the difference between the winner's and loser's combat score. If the total is greater than the unit's Leadership (Ld) value then the unit is broken. Broken units will turn tail and flee once all combat on the entire battlefield has been worked out. Until all combat has been worked out, simply turn a few of the rear rank models round to remind you that the unit is broken.
For example: A unit of EIf Archers is fighting a unit of Goblin Spearmen. The Goblins inflict 3 wounds on the Elves, and the Elves inflict 4 wounds on the Goblins. However, the Goblin player has four complete ranks in his formation, each extra rank adding +1 to his score, and his troops outnumber the Elves, adding another +1. This gives him 3 + 4 = 7 points against the Elves' score of 4.
The Elves have therefore lost the combat, even though they have caused more casualties - the vast numbers of Goblins pressing from the back have overwhelmed them. The Elves must therefore take a Break test adding +3 to their dice score, because the difference between the scores is 3. Elves have a good Leadership value (8) but with the extra + 3 modifier on the dice, the player will have to roll 5 or less to stand and fight. The player rolls 2D6 and scores 7. The +3 modifier brings his total to 10 which is greater than the unit's Leadership, so the Elves are broken.