A piece attacks through an intervening piece
X-ray: one piece attacks a target through another piece. Trade off the intervening piece and the attack lands. The classic shape is two rooks on the same file, where the back rook x-rays the target through the front rook.
Capture the rook on e8 with check. Black is forced to recapture with the e7 rook.
Go on then, take it.
3 more clean runs (at any point) to reach Recall.
White to move. Two white rooks on the e-file and d-file. The d8 rook attacks the e8 rook directly. The e3 rook attacks e8 through e7.
3 more clean runs (at any point) to reach Recall.