Box-line reduction is the inverse of pointing pairs. If every possible home for a digit in one row or column sits inside the same 3×3 box, then that digit cannot appear elsewhere in the box.
Look for a row or column where a digit is trapped inside one box. Remove that digit from the rest of that box, then place the newly forced value.
The first move in this lesson is a box-line reduction. After that, the board opens into familiar singles and subset patterns.