When one additional digit k is present in one of the cells, then a unique corner has been found and k may be entered there so as to avoid the deadly condition.
When one additional digit k is present in two adjacent corners (a unique side) of the unique rectangle, then those corners form a conjugate pair of cells for digit k, and k may be eliminated from possibilities in the row or column containing the two corner cells. Additionally, if those corners are in the same box then k may be eliminated from other cells in the box. Other exploitations of the unique rectangle are known, but uncommon.
This deadly pattern is but one of the many possible. Consider a completed Sudoku grid - all 81 entries are present. The clues must be chosen so as to leave no deadly pattern exposed or the solution will not be unique. If we were to exchange of all the 1 digits with all the 9 digits, we have a deadly pattern of 18 cells, and one of the cells must have a clue to prohibit the free exchange of the two digits. (The deadly patterns are unavoidable sets for Sudoku. There are hundreds of these overlapping sets for each completed grid, and each set must have at least one clue to preserve uniqueness.)
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