Look at where a digit can still go inside one 3×3 box. If every remaining spot happens to sit in the same row or the same column, the box has made a promise: the digit will be placed on that line, inside that box. The rest of the line — the six cells running through the neighbouring boxes — can drop the digit entirely.
Box 1 confines digit 5 to row 2 (blue) — the 5s elsewhere in row 2 (red) are eliminated.
In the top-left box, digit 5 is blocked from row 1 by the 5 at r1c7 and from row 3 by the 5 at r3c5. Whatever survives sits in row 2 — say r2c1 and r2c3. The box therefore promises its 5 to row 2, and the 5s pencilled further along that row, at r2c5 or r2c8, are dead.
The promise only holds while ALL of the digit's spots in the box share the line. One overlooked candidate on another row of the box and the pointing pair evaporates — recheck the box before erasing outside it.