Whenever a card leaves your opponent's scrapyard, remove the top card of their hold from the game.
11
Explained
Whenever a card leaves your opponent's scrapyard, remove the top card of their hold from the game.301.11
Once for each card, however it leaves: Salvage, Resurface, Refuel, a shuffle into the hold, or removed from the game, by either player. It happens at once (301.11). The cards go from the top of their hold straight out of the game, face up. It isn't a dig: nothing that counts digs sees it, and an empty hold loses nothing (403.5).