Box/Line Reduction
When all candidates for a digit in a row or column fall within the same block, that digit can be eliminated from other cells in that block.
Box/Line Reduction applies when all candidates for a specific digit in a row or column are confined to a single block. The digit can then be eliminated from other cells in that block (those not in the same row/column). This is the reverse operation of Pointing Pair.
Relationship to Pointing Pair
Pointing Pair eliminates in the "block to row/column" direction, while Box/Line Reduction eliminates in the "row/column to block" direction. The logical basis is the same - constraint intersection - but the starting point of the scan differs. Being aware of both directions ensures you never miss an elimination opportunity.
Usage at Hard Level
Box/Line Reduction is frequently needed at Hard difficulty. Along with Naked Pairs, it is one of the key techniques for breaking through the Hard-level barrier. With accurate pencil marks, it is relatively easy to spot.