A square with two options, the pivot, sees two others that each share one of its options and a third number. Whichever way the pivot goes, one of them takes that third number, so a square that sees both can't have it.
123456789
123456789
3
9
6
8
7
1
5
6
1
5
3
2
7
8
8
24
7
5
49
1
3
6
24
6
3
29
5
1
8
7
5
7
3
8
1
4
6
8
1
7
6
5
3
4
7
5
8
3
6
1
3
1
8
7
1
7
8
3
Pivot and wingsLoses a pencil mark
Also called XY-Wing
Some books call it an XY-Wing: the pivot holds X and Y, and each wing holds one of them with the shared Z.
How to spot one
Look at squares with two options. Find one whose two numbers each appear, with the same partner, in squares it can see.
What it gives you
One or two removals where the wings' lines cross. They turn up on tough boards.
Practise Y-Wing
Boards that need it, with the owl's hints when you're stuck.