Search for Pairs of Numbers
It's easiest to describe the search of pairs of numbers by an example:
 A sparse Sudoku
This Sudoku does not have a uniq solution, but it is a good example nontheless.
When you try normal crosshatching you don't find a
number, but you find that the 2
and 8
have
only two possible cells in the upper left block.

The
2
and8
have only two possible cells each.
You should mark this in your Sudoku because it is usefull later on.
 The places for the two numbers are marked.
The 2
and 8
have to be in these two
cells, therefore no other numbers can be in these cells.
 The two cells are blocked for all other numbers.
Knowing that you can again use crosshatching:

You can find a place for the
4
using crosshatching.
The 4
previously had three possible positions in the
upper left 3x3 block, now only of of them is left:

The Sudoku with the newly found
4
This technique is very usefull when using pen and paper to solve a Sudoku.
It works with triples instead of pairs as well, if there the same three cells are possible for all three numbers.
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