Take two squares that see each other and list the ways they could be filled together. A pair that would leave a third square with nothing is ruled out, and a number no surviving pair uses goes.
123456789
123456789
6
4
1
59
8
2
39
2
7
35
6
349
8
1
5
1
8
2
4
9
7
5
3
1
8
1
7
4
8
5
8
5
6
1
7
7
9
8
3
1
5
8
2
3
3
6
8
The pairSquares they both seeLoses a pencil mark
Why it works
The two squares hold two numbers between them. Any pair that takes both of a neighbour's last options is impossible, so only the pairs that leave every neighbour something survive.
How to spot one
Look for two squares that see each other, each with few options, beside two-option squares that see them both. List the pairs; there are fewer than it sounds.
What it gives you
One removal, usually. It comes after the shapes, because it means listing combinations rather than spotting one.
Practise Aligned pair exclusion
Boards that need it, with the owl's hints when you're stuck.