Bivalue Universal Grave type 2

If the value 1 were removed from the cells F7 and I9, the Sudoku would exhibit a Bivalue Universal Grave pattern (or BUG).

In a BUG, each value that remains in a row, column or box has exactly two possible positions in that row, column or box; and each empty cell has exactly two potential values. A Sudoku having a BUG has zero, two or more valid solutions.

Because a valid Sudoku has exactly one solution, the BUG cannot be part of it. The only way to avoid the BUG is if at least one of the cells F7 or I9 contains the value 1. Other possible positions of this value can therefore be removed from other cells sharing a row, column or box with F7 and I9.