This card gets better, when you know the scenario and "ambush" fixed spawns (advancing act or agenda).
It doesn't matter which player stays behind. This allows the parties fighter to "protect" a clue gatherer, as long as the gatherer stays at same location.
Even if it does not trigger, it may safe a lot of time if the gatherer knows he is safe for 1-2 rounds while the fighter saves someone else or hunts specific enemy (Smite the Wicked).
It's very useful when there's lots of clues on one location, like in bigger parties (3-4 players) or special locations (The Hidden Chamber).
It also helps on higher difficulties, as the damage is an "auto success".