The decoy is a move--often a sacrifice but not necessarily--designed to force a piece to a specific square or line for the purpose of enabling a combination. This is different--by the way--from deflection which gets a piece away from a specific place where it is performing some duty, but it doesn't particularly matter where it ends up. The term "Overworked Piece" is also used for both Deflection and Decoy. 

White plays and wins. To see the answer select the space between the quotation marks.

"1. Qe7+ Kg8 2. Qe8+ Rxe8 3. Rxe8 mate"

 

The first move is the decoy move forcing the King to move behind the Pawns that normally guard the King--The Praetorian Guards--making possible a Back Rank Mate.

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Decoy--2

Decoy FEN

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