How to solve a finned X-Wing in Sudoku
A finned X-Wing is an X-Wing with an extra candidate (a fin) in one box, which still clears the digit from that box.
How it works
It looks like an X-Wing with one extra cell, the fin, sharing a box with one of the corners. Either the fin holds the digit, which clears the digit from the rest of its box, or it does not, and the X-Wing works as normal.
In both cases, the digit can be removed from the cells in the X-Wing's columns that lie in the fin's box.
How to spot it
You find it as an almost-X-Wing: two rows that nearly line up on a digit, with one spare cell in the same box as a corner.
A worked example
This is a step from a real puzzle, a moment where the next move needs a finned X-Wing.
F are the fish cells, R are the fins (the extra cells, all in one box). Struck-through digits are the ones that can be removed.
7 is almost limited to 2 columns across 2 rows (R2C1, R2C8, R6C8), with the extra cell R6C2 (the fin) in box 4. If the fin is 7, it clears 7 from the rest of box 4. If not, the 2 rows use up 7 in those columns. Either way, remove 7 from R4C1, R5C1.
Where it fits in Rocksled Sudoku
Puzzles that need a finned X-Wing are never rated below Expert. Each step of this kind adds 68 points to a puzzle’s difficulty score. When you ask for a hint that needs it, the game names the technique and shows the cells involved. See how to play for the rest of the controls.