Lesson
White to move. Main line: 1. Qe8+ Rxe8 2. Rxe8#
Qe8 gives check and attacks nothing less than the back rank itself. The rook must capture — it is the only legal reply — and in doing so it steps onto the very square where the white rook mates.
Deflection means forcing a defender to leave its duty. Count the defenders of the mating square: if there is exactly one and you can force it away — even by sacrificing your queen — the material never matters.