Hidden Pairs, Trio's and Quads
This is an excellent way to reduce the number of possible solutions. The method is very easy
Start by looking for unique identical pairs which appear only twice in a row, column or box. This means that the pair of numbers you have found have to be on one of cells, so all other number can be eliminated.
First Example :
In the example above, I have found the number pair (3,8) in the first row in the cells H1 and I1.
Because the numbers 3 and 8 do not appear anywhere else in this row, the only solution for the cells H1 and I1 is either a number 3 or a number 8. This means that all other solutions can be eliminated.
After elimination the content of cell H1 will be (3,8) and for cell I1 also (3,8).