If a defeated unit has higher Unit Strength than its enemy, it takes its Break test on its unmodified Leadership.
When at war, there's definitely something to be said for having vast numbers at your disposal. Not only will your warriors be worried far less by the odd fallen comrade (there's plenty more where they came from) but they'll also take heart from being more numerous than the enemy. To represent this in our games, we have something called the Steadfast rule.
A unit is considered to be Steadfast if it has a higher Unit Strength than its enemy and at least Unit Strength 10. In addition, it must also have a rank of 5 or more models (including the front rank). Disrupted units and Skirmishers (see Special Rules chapter) cannot be Steadfast.
Steadfast units always take Break tests on their own Leadership characteristic, ignoring any negative close combat resolution modifiers. It doesn't matter whether they have been beaten by 1 point or by 100 points, they still use their normal Leadership value. However, remember that units that are Disrupted cannot be Steadfast.