At the start of your Command phase, select one unit from your opponent's army to be your focus of hatred. Until the start of your next Command phase, each time a Heretic Astartes model from your army (excluding Damned models) makes an attack that targets your focus of hatred, you can re-roll the Hit roll.
That is the whole rule, and its virtue is that it has no other condition at all. No keyword, no phase, no range, no objective, no pact. If the target is your focus of hatred, everything you own re-rolls hits against it.
An official FAQ confirms you can select a unit that is embarked within a Transport, which matters more than it sounds: you can name a squad while it is still inside a Rhino and be ready for the turn it disembarks.
The skill is target selection. A full re-roll on hits is worth most against a unit you can genuinely kill in one turn, and worth almost nothing against something you can only chip. Pick the unit your army is about to focus anyway.