Decoy is a tactic where you force a piece to a square that it doesn't want to be on. This can be used to remove a defender, lure a piece into a fork or even lure a king into checkmate.
Can you find the forcing moves that win Black's queen by forking the king and queen? The first step is to force Black's king to a forking square...
Since ...Ka7 allows the Nc6 fork, Kb7 is forced. How can you decoy Black's king to a forking square?
Black's king is forced to a forking square. Win Black's queen!