When a box's places for a number all lie on one row and one column of the box, the box works like a hinge. Paired with a line where the number has two places, it rules the number out of one square.
123456789
123456789
8
1
2389
5
39
4
4
9
7
1
6
4
9
3
2
4
9
6
3
2
5
8
3
2
1
7
8
9
5
4
6
4
8
359
2
7
1
7
2
9
8
8
9
69
7
1
4
The box's two placesTwo places in row 2Loses a pencil mark
The name
The box's squares off the cross can't hold the number. That empty patch gives the technique its name.
How to spot one
Look for a box where a number's places sit on one row and one column. Follow them out of the box to a line with two places.
What it gives you
One square loses the number. It's a short chain, with the box doing the work of one link.
Practise Empty rectangle
Boards that need it, with the owl's hints when you're stuck.