 Anand should have answered 27...b1=Q+ with 28.Bf1.ChessBase gives the best line as 28.Bf1 Qd1 29.Rh4 Qh5 (Black must sacrifice his new queen in order to stave off checkmate) 30.Nxh5 gxh5 31.Rxh5 Bf5 32.Bh3 Bg6 33.e6 Nxf6 34.gxf6 Qxf6 35.Rf5 Qxe6 36.Re5 Qd6, which is probably a draw.Instead, Anand blundered with 28.Nf1 and resigned after 28...Qe1, since after 29.Rh4 Qxh4 30.Qxh4, Black emerges a rook up.