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. 4,5,6 in column 9 are [1,2,5,6][1,2,5,6][1,2,6].
In [R3C9]’s [5,8,9], a 1, 2, or 6 can’t be entered.
Also, in [R8C9]’s [5,8,9], a 1, 2, or 6 can’t be entered.
It appears that a 1 might fit in [R9C9], but since 1 can only go in row  in column , it is impossible.
Hence, a triple of [1,2,6] arises in 4,5,6 of column 9.
Therefore, [1,2,6] can’t be entered in the other cells of this box. Hence, a  in [R5C8] and a  in [R6C8] are confirmed.
 in [R7C7] is also confirmed.
It’s not quite hidden, but a triple of 1,7,8 arises between [R4C4], [R5C4] and [R8C4]. The 1 will have to be in either [R4C4] or [R5C4], so it will not be in [R5C6] or [R6C6].
Hence, 1 in [R9C6] 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, [R1C8] and [R1C9] 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.