X-ray

An x-ray is a line attack that counts THROUGH an enemy piece. A rook on f2, an enemy rook on f6, an enemy rook on f8: the piece in the middle is not a shield. The moment it moves, or is forced to capture on f8, the rook on f2 reaches all the way up the file. The attack was there the whole time; the blocker only postponed it.

What makes an x-ray WORK is a forcing move on the far square. Capture on f8 with a piece the blocker must take back, and the blocker lands on the square you were x-raying, with your line piece now hitting it directly. The same geometry defends: a queen on e7 can protect h4 through an enemy queen on g5, because whatever captures on h4 is captured back.

The way to spot one is to count attackers on a square including the pieces behind an enemy piece on the same line. Most players stop counting at the first piece they meet. If that piece is the opponent's and it can be forced to move or to capture, the piece behind it is an attacker too, and the square may be attacked more times than it is defended.

The x-ray hides in back-rank mates above all: the rook that recaptures on f8 is the rook that gets mated there, because the rook behind it was counting on the file the whole time.