This is a technique that uses intersections between lines and boxes to eliminate candidates. It is a basic technique that can be applied when all candidates for a digit in a house is contained within the intersection of another house. In this case we can eliminate all the remaining candidates from the second house that fall outside the intersection.
There are 2 types of this technique, and the terms Pointing and Claiming/Box-Line reduction are often used to describe these types.
Type 1 (Pointing)
This type applies when all the candidates for a digit within one box is contained in one line. Then all the remaining candidates in that line that is not contained within the box can be removed.
This example shows an example of Type 1. The digit 6 in box 1 can only be placed in column 2, this means that the digit 6 can not be placed in any other cells in column 2 and therefore can be eliminated from the cells r4c2 and r6c2
Type 2 (Claiming/Box-Line reduction)
It’s the opposite of type 1. When all candidates in a specific line is contained within one box, then all the candidates within that box that is not on the same line can be removed.
Locked Pair/Triple
An extension of the locked candidate are the locked pair/triple. These are Naked Pair/Triple contained within one intersection, so the remaining candidates for all the locked candidates can be removed from both intersected houses, making them both a Type 1 and Type 2.
As the subset have to be contained in one intersection, a Naked Quad can never be locked.
