Decoy is a piece sacrifice in chess made in order to get an advantage by forcing the opponent piece to change its position.
Let's see an example.
1. Qh7 Queen sacrifice by White. Black King is obligated to capture the Queen.
If not 1. ... Kf8 2. Qh8 checkmate.
After King captures the Queen , Discovered check by Bishop . Rook controls the escape squares of the opponent King.
1. ... Kxh7 2. Re8 mate.