XYZ-Wing
A middle cell with candidates ABC sees two wings, AC and BC. One of the three cells must be C, so a cell that sees all three can’t be C.
Why it works
On the board above:
r4c5 can be 1, 8 or 2. r8c5 is 1 or 2, and r4c6 is 8 or 2.
If r4c5 is 1, then r8c5 is 2. If it is 8, then r4c6 is 2. If it is 2, it is 2 itself.
So one of the three cells is 2. r5c5 sees all three, so it can’t be 2.
How to spot it
Find a middle cell with three candidates
Call them A, B and C.
Find two wings it sees
Each wing has two of the middle’s digits, and both wings hold C.
Look for cells that see all three
These are almost always in the middle cell’s box.
Clear C from them
Only those cells lose C.
Practice
Real boards from the solver, with every pencil mark filled in. Spot it, then use it, then find it cold.
Loading the boards…
Try it in a real puzzle
Each of these needs an XYZ-Wing to finish. It opens in the player.
Tips
Fewer cells to clear
Because the cleared cell must see all three, an XYZ-Wing usually clears only one or two cells.
Start in a box
Look for a three-mark cell that shares a box with a two-mark wing.