A sudoku has one answer. Four squares at the corners of a rectangle across two boxes can't all hold the same pair, because the two numbers could swap and give a second answer. When three corners hold only that pair, the fourth must be something else.
123456789
123456789
9
4
3
7
2
8
6
1
5
26
26
5
1
3
4
8
8
1
7
6
9
5
5
1
2
4
3
8
267
26
4
8
9
5
3
9
8
5
6
4
2
4
8
9
3
6
5
5
7
6
8
3
1
3
2
5
7
8
6
Three corners with the pairLoses a pencil mark
Why two boxes
Across exactly two boxes, every row, column and box the corners touch holds the swapped pair the same way, so both versions fit. Across four boxes a swap could break a box.
Only for proper puzzles
This relies on the puzzle having exactly one answer. Every board in the game does; a puzzle with several answers would break it.
How to spot one
Look for three squares with the same pair at three corners of a rectangle in two boxes. The fourth corner is where the extra numbers are.
Practise Unique rectangle
Boards that need it, with the owl's hints when you're stuck.