1.Re3! Black has difficulty watching both the g8 and d5 squares. If he loses control of either, White has an N mate waiting. 1...Qg2 The only move. 2.Bg4! Cuts off the Q from g8, and takes advantage of a selfblock after 2...Qxg4? 3.Nd5+ Kf5 4.Re5#. 2...Qa8 3.Re5! The Bishop holds f3, and the Rook covers f5 in case Black decoys the wB away. 3...Qd8 4.Bd7! Qa8 5.Re4!! And White has restored the diagram position, but with Black to move. Black can no longer hold both the g8 and d5 squares, and will be mated next move.