If a character is hit by a template weapon or spell that uses a template, a comrade will shout a warning or physically push them clear of incoming harm and suffer the hit themself instead. This happens automatically, and no roll is required, nor is this considered as a "save" for rules purposes.
"Look Out, Sir!" cannot be used if there are less than five rank-and-file models (including command group) left in the unit. This only applies to Characters with the same troop type as the unit.