The key 1.Kh1 vacates a square for the queen while taking care to avoid a potential rook check on the second rank after 1...bxa2. The threat 2.Bf4 opens a line for 3.Qg1, while closing the rook guard of f2. The two defences show similar play, with a line being opened for the queen and a rook line being closed.
1...d5 2.Bf5 threat 3.Qg4;
1...Bxb2 2.Rc6 threat 3.Qa7. If 2...Se7 3.Qxf6, or 2...Rxb5,Rc4 3.R(x)c4.
Dafydd Johnston: Nice symmetry between the white rook and bishop which both clear a line for the queen and shut off the black rook in anticipation of the unpin. All that is needed from the key is clearance of the queen's destination square. An excellent problem by one of the best writers on the subject.