Hidden Pair, Triplet, Quad...

A related technique is known as “hidden pair” (triplet, quad…).

The pattern is very similar to naked subsets, but instead of affecting other cells with the same row, column or block, you can eliminate candidates from the very same squares that monopolize the numbers.

For example, consider a row that has the following candidates:

You can see that there are only three cells that have any of the candidates (1, 3 7).

These three squares monopolize those three numbers, so, obviously, these three cells cannot hold any other value, meaning we can eliminate the numbers I’ve crossed out!

After crossing out the numbers, we're left with:

You may notice that one of the cells doesn't have 3 as a candidate, but this makes no difference at all. The important point is that there are only three cells in which those three numbers appear at all.

In the Sudoku puzzle below, the highlighted cells have the hidden pair (3, 5). The crossed-out numbers can be removed from contention. Can you find a naked triplet in the same row?