Looking for Hidden pairs is a great way to open the board up to the other tests. Consider this top centre box below. There are two 5/8s hidden in in the squares at the top of the 3 x 3 box. They are marked by the green squares. We know these are the only possible positions for 5 and 8 since the rest of the board excludes the other squares in the box.
Since 5 AND 8 must exist in those two squares the two 1s, the 7 and the 9 cannot exist there. So we remove them. This reveals the hidden pair.
The knock on effect of this is to leave just one 7 in the box. We can make that the solution of that square and probably complete the rest of the board:
Exposing the pairs like this is essential for the Box/Line Reduction and the Remote Pairs - or at least makes them identifiable as appropriate strategies.
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