White wants to get the g1 rook to c5, and so must use the a2 bishop to shield his king. 1.Kb1 threatens 2.Ka1 3.Bb1 4.Rc1 and 5.Rc5. Black’s only defence is to march his king in the opposite direction to use the g6 pawn as a shield. 1…Kf5 2.Ka1 Kg5 3.Bb1 Kh5. Now White changes tack, and 4.Bg7 leaves Black the choice of 4…Kg5 5.Rb5 or 4…g5 5.Rh6.