In a box, a digit fits only along one line: it goes from the rest of that line.
What to look for
Take one box and one digit. If every cell of the box where that digit can still go lies in the same row (or the same column), those two or three cells point along that line.
Why it works
The box needs the digit somewhere, and every choice is in that row. So the row’s copy of the digit is inside this box, and it cannot also be in the row’s cells outside it: cross it out there.
Practise it
Puzzles that need it: Medium sudoku
On a real puzzle
Pointing
In box 2, 2 can only be in column 6.
So 2 can go from the highlighted cells.
The cells that show the pattern are in amber, the cells it changes in green, and the candidates it removes are circled.