As soon as a model with this rule is reduced to 1 Wound he gets a Ward save (3+) for the rest of the battle. This has no effect on attacks that would outright kill him before he is reduced to 1 Wound, like a spell or attack with Multiple Wounds. If he were to suffer multiple Wounds in a single phase, roll one save at a time (if he has any) for each Wound suffered until his Ward save takes effect.