Figure 6.1.7.4[Black to move]

Now the current idea from Black's side. White’s king is at the edge of its pawn cover with Black’s power aimed down the f-file. If the king could be pushed back into the corner Black would have two attackers aimed at f1 against just one defender (the rook on e1). The classic way to give the king such a push is with diagonal pressure, and Black has two pieces available for that purpose: a queen and a bishop. He needs them both, because White has a bishop on b2 poised for counterattack—and indeed ready to take Black’s queen immediately. Thus Black starts with Qd4+. White must either move his king to h1 or play BxQ, so he goes the latter route; but now Black has BxB+, and Kh1 is White’s only remaining option. The rest is routine. Black plays Rf1+, forcing RxR; then RxR mates.