At the start of the first battle round, select two objectives to be your foundations. Place a circular foundation marker in each one. When you draw a line from the closest part of one foundation marker to the closest part of the other, if any part of a model’s base (or any part of a model’s hull, for a model without a base) crosses that line, that model’s unit is said to be on your defensive line.
While an IMPERIAL KNIGHTS unit from your army is on your defensive line, that unit has the following ability:
Against the Horde: Each time a model in this unit makes an attack that targets a visible enemy unit, you can ignore any or all modifiers to the Hit roll, and weapons equipped by models in this unit have the ability.