Hidden pair
Two values confined to two cells lock those cells
How to use it
Two symbols whose possible homes inside one unit are exactly the same two cells. Whether the arrangement is one way or the other, those two cells hold the two symbols — so every other candidate in their marks is dead weight: strike it.
The scan is the hidden-single scan extended: for each unit, group symbols by their host sets; two symbols sharing a two-cell host set form a hidden pair. The payoff arrives afterwards, when the reduced cells often collapse to naked pairs or singles.
Why it is sound
If symbols x and y are possible in a unit only at cells a and b, then a and b carry x and y in some order in every solution. Any other candidate of a or b would leave one of the two symbols without a home, so restricting both cells to {x, y} discards no solution.
A worked example
In column 3, ask where the letters C and F can still go. The givens, their peers and the pair work so far rule C out of every empty cell except r1c3 and r6c3 — and F, likewise, survives only at those same two cells. Two letters, two cells: r1c3 and r6c3 carry C and F in some order. Everything else in those cells' marks is dead weight: D is struck from r1c3 and G from r6c3. Hidden pairs are found by grouping symbols by their host sets, not by scanning cells — and the reductions they deliver often leave naked pairs or singles behind.