If a unit rolls double 1 for its Break test, it passes it, regardless of any other modifiers.

Occasionally, in the middle of a battle, even the humblest regiment becomes filled with steely courage and discipline, deciding to stand their ground, no matter the odds! Such unpredictable occurrences are represented in the game by the Insane Courage rule. This simply means that if a unit rolls a double 1 for its Break test, it will stand its ground, regardless of how badly it has lost the fight.

For example: A unit of Night Goblins is charged in the flank by a unit of Chaos Knights. The combat is resolved and the Chaos Knights win the fight by 10 points. The Night Goblins' Leadership of 5 means that they would need to roll -5 or less (assuming, of course, that they are not Steadfast), which is obviously impossible. There still is a point in rolling the dice for the Night Goblins though, because there is a remote chance of rolling a natural, unmodified, double 1, meaning that the Goblins would brave the onslaught & heroically (foolishly?) stand their ground.