A swordfish is the X-Wing scaled up from two lines to three. Take one digit: if three rows each confine their candidates for it to the same three columns, then the digit’s three placements in those rows must occupy three of the crossing cells — one per row, one per column. The three columns are thereby claimed, and the digit is struck from every other cell in them.
To spot one, track a single digit across the grid and write down which columns it can still occupy in each row. A row reduced to two or three columns is a candidate line; three such rows drawing on the same trio of columns complete the fish. The pattern does not need all nine crossings live — this board’s fish runs on seven.
It works by the same exchange as the X-Wing: each of the three rows must place the digit in one of the three columns, and no two rows may share a column. That spends each column’s copy of the digit inside the pattern, so nothing outside those three rows can hold it. (Transposed, three columns can cage three rows the same way.)
On the board
Follow the digit 9. Row 1 can only hold it at columns 2 and 8; row 5 at columns 2, 8, and 9; row 7 at columns 8 and 9. Three rows, and every candidate falls in the same three columns — 2, 8, and 9. However the three 9s end up arranged, those columns are spent, so the 9 is struck from the columns’ other cells: R3C2, R2C8, R4C8, R3C9, and R4C9 all lose a pencil mark.
This is the exact board the app's interactive Swordfish demo uses — step 6 of its 62-step walkthrough, drawn here with its live pencil marks.
