Wings
How it works
Four cells carrying only four candidate digits between them, spread across a box and a line rather than sitting in one unit — that would just be a Naked Quad. Three of those digits can be used only once each inside the set, because every cell holding them sees every other one; three digits cannot fill four cells, so the fourth digit Z has to be placed somewhere in the set. Any cell outside it that sees every set cell holding Z loses Z.
Try it
Look at Row 4, Column 4, Row 4, Column 6, Row 5, Column 2, and Row 5, Column 6 — four cells sharing a box and a line. Let's pencil them in, then see why 3 is trapped inside them.
4
1
7
8
6
5
5
7
3
4
1
6
8
1
5
4
3
7
2
9
9
7
8
4
8
6
5
2
7
1
7
2
3
6
7
6
5
3
2
9
4
1
3
5
6
7
8
3
1
8
7
6
9
4
5
2