4-cell Kakuro run summing to 12

Only 2 digit sets sum to 12 across 4 cells. That makes this run useful for cross-reference deductions: a digit you can rule out from one of the cells (because the perpendicular run forbids it) often forces the entire combination.

Locked digits: 1, 2. These digits each appear in every valid combination, so they each must occupy one cell of the run; useful when a perpendicular run rules any of them out of a candidate cell.

Forbidden digits: 7, 8, 9. None of these digits appears in any valid combination, so you can rule them out of the run's cells immediately.