Two groups of squares, each with one more number than it has squares. When they can't both hold a shared number, a second shared number must land in one of them, so a square that sees every copy of it in both can't have it.
123456789
123456789
7
9
478
1278
178
128
3
6
2
4
9
6
1
1
47
6
3
5
2
6
5
8
3
6
5
7
2
2
5
1
8
2
4
1
2
5
3
7
9
6
5
3
2
7
1
Four squares, five numbersOne square: 4 or 7Loses a pencil mark
Almost locked sets
A group of squares in one row, column or box with one more number than squares is almost locked: take one number away and the rest are locked in. A single square with two options counts.
How to spot one
Look for a two-option square and a small group nearby that share two numbers, where every copy of one of them can see every copy in the other group.
What it gives you
Removals of the second shared number, in squares that see all of its copies in both groups.
Practise ALS-XZ
Boards that need it, with the owl's hints when you're stuck.