1.Nf3 f6 2.e3 e6 3.Nd4 c6 4.Nb5 cxb5 5.Qh5+ g6 6.Qb5

Black cannot play 6...Kf7 since 7.Bc4 Kg7 8.Bd3 f5 9.Qxf5 and black is lost, so 6...Nc6 is the only move.