Two squares with the same pair of options, joined by a line where one of those numbers has only two places. The twins can't both be that number, so a square that sees both can't have the other.
123456789
123456789
5
2
7
9
3
8
4
2
6
3
5
3
6
1
9
8
3
4
6
12
5
9
7
6
3
4
1
2
8
6
3
5
3
9
12
5
6
9
12
3
7
8
3
8
7
1
5
The twinsLoses a pencil mark
Why it works
Each twin blocks one end of the link. Both twins can't be 1, so one is 2, and whichever it is, the squares that see both can't be 2.
How to spot one
Find two squares with the same pair that can't see each other. Then look for a line where one of the pair has exactly two places, one seen by each twin.
What it gives you
Removals in the squares that see both twins. It's like a Y-Wing, with a line doing the pivot's job.
Practise W-Wing
Boards that need it, with the owl's hints when you're stuck.