Open line to guard escape square (2 moves)
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This combination is best explained with some examples. It often includes surprising and beautiful sacrifices:
Examples
Consider that 1...Ng3+? is not checkmate, because the king still has the escape square h2. Black can force checkmate with 1...Rh3+! and after White's only move 2.Bxh3 the line from f2 to h2 is opened for the queen to guard the escape square h2, and 2...Ng3# is now checkmate.
Note that 1.Bh7+? would not be checkmate, because the king can still escape to h8.
Jan-Krzysztof Duda played the spectacular queen sacrifice 1.Qxh6!!, threatening checkmate on g7 and h7.
If Black captures the queen with 1...gxh6, the diagonal from d4 to h8 is opened for the bishop to guard the escape square h8.
2.Bh7# is now checkmate.