Four squares that between them hold only four numbers, where every number but one can fill at most one of them. That one number must be in the group, so a square that sees all its copies can't have it.
123456789
123456789
9
8
1
5
5
9
123
27
6
23
8
4
8
3
5
24
124
9
7
5
9
6
1
5
7
4
2
2
7
6
9
5
3
8
6
4
5
8
1
7
9
4
6
9
8
5
3
8
7
1
5
4
2
6
The four squaresLoses a pencil mark
The odd one out
Every number in the group but one sits in squares that all see each other. That one is the number the group is sure to hold.
How to spot one
Look for four squares, in a box and a line through it, whose options come to four numbers. The number whose squares don't all see each other is the one that clears.
What it gives you
Usually one removal, in a square that sees every copy of that number in the group.
Practise WXYZ-Wing
Boards that need it, with the owl's hints when you're stuck.