Like the naked pairs, this method involves looking for pairs like [1,2][1,2], in cases where these pairs are hidden. Since they are hidden numbers, they are harder to find, but when you find them, [1,2] can be eliminated as candidates from other cells.
Lets have a look at the center-right box. DEF in column 9 are [1,2,5,6][1,2,5,6][1,2,6].
In C9’s [5,8,9], a 1, 2, or 6 can’t be entered.
Also, in H9’s [5,8,9], a 1, 2, or 6 can’t be entered.
It appears that a 1 might fit in I9, but since 1 can only go in row G in column 7, it is impossible.
Hence, a triple of [1,2,6] arises in DEF of column 9.
Therefore, [1,2,6] can’t be entered in the other cells of this box. Hence, a  in E8 and a  in F8 are confirmed.
 in G7 is also confirmed.
It’s not quite hidden, but a triple of 1,7,8 arises between D4, E4 and H4. The 1 will have to be in either D4 or E4, so it will not be in E6 or F6.
Hence, 1 in I6 is confirmed.
As seen in the diagram below, if 1,2 is in the upper left and upper middle, and there is also 1,2 in column 7, A8 and A9 in the upper right box will become [1,2][1,2]
Since a [1,2] pair is now in the upper right box, 1 and 2 can’t be entered in the other cells.
Quads are made up of four numbers in the same way, but we couldn’t find a specific example for this. It will appear as if there are many candidates, but if a number can’t be entered in any of the other cells, this technique will work.
It is a bit difficult, but it is an effective technique when you get stuck.