Lesson
White to move. Main line: 1. Qxd7 Rxd7 2. Rxd7
1.Qxd7 removes Black’s queen and forces 1...Rxd7. The black rook lands on the same square, but White’s rook on d1 has been attacking through the white queen all along and now recaptures with 2.Rxd7.
The final material count is the point: queens and Black’s rook have disappeared, while White keeps a rook. The first capture is safe because the piece behind the queen completes the exchange sequence.
An x-ray attack is pressure through an intervening piece. On files and diagonals, mentally remove the front piece and ask what the rook, bishop or queen behind it would attack; forcing trades often make that hidden pressure real.