Clearance for checkmate (2 moves)
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In this level you move one piece out of the way for another piece to give checkmate on a subsequent move.
White has a "battery" of rooks on the f-file. In a battery, if one piece moves, it clears the way for the other piece to follow. White wins with Rf8+, clearing the way for the 2nd rook on f1. After Kg7, White can checkmate with R1f7#.
A clearance can also be a quiet move, as in the following example:
Black's king was driven all the way to the first rank, where it does not have any escape squares. White can win with the quiet move h4, clearing the way for Qh3#. This is a mating net, as Black has no way to prevent checkmate.
Sometimes the clearing move can be a sacrifice:
White can win with the knight sacrifice Nb3+, clearing the way for the queen to b2. After Bxb3 (and after Kb1), Black can checkmate with Qb2#.