16...Bxb3 17.cxb3?? Diagram
this allows a really good move for black
what is it ?
it is a DOUBLE ATTACK
also a DISCOVERED ATTACK - particularly when the KING is being attacked TOO 17...Nd4+ you have to protect the King FIRST *
This was played less than a week ago in the United States Championshiip 2010 and what you will see is use of the IMAGINATION to see wheter a certain move is worth playing AT ALL like last week some moves can involve TRAPPING PIECES or putting them OUT OF PLAY - what do you think we mean by those two phrases ???? 1.e4 c5 2.Nf3 Nc6 3.d4 cxd4 4.Nxd4 Qb6 5.Nb3 Nf6 6.Nc3 e6 7.Qe2 Bb4 8.Bd2 0-0 9.a3 Be7 10.0-0-0 d5 11.Be3 Qc7 12.exd5 Nxd5 13.Nxd5 exd5 14.g3 Bf5 15.Bg2 Rac8 Diagram
and here white has a choice of quite a few moves - but tempting is capturing the black pawn on d5 by the Rook on d1
is this a good move or not ????
we have our IMAGINATION or VISUALISATION to helkp us out 16.Bf4 do this FIRST was what the white player chose [16.Bxd5 Nb4 Diagram 
; 16.Rxd5 Be6 17.Rb5 a6 18.Rb6 VERY VERY out of play - but the computer prefers it in the short term
it seems that white will have to exchange Rook for Knight or a Bishop (5 Versus 3 points) which is costly(18.Rc5 Bxc5 although this is tempting t is too simple - THERE IS BETTER(18...Bxb3 19.cxb3?? Nd4 DOUBLE ATTACK with King being one of the pieces attacked - as the previous position showed (19...Bxc5 20.Bxc5 Nd4 ORDER OF CAPTURING is an important area we use our IMAGINATION and VISUALISATION with 21.Qd3 Qxc5+) 20.Rxc7 Nxe2+ 21.Kd1 Rxc7) ) ] 16...Qb6 17.Rxd5 Bxc2 [17...Be6 18.Rb5 this is probably enough to show the difference between capturing with the Rook FIRST and PREPARING for it] 18.Qxc2 Na5 19.Nxa5 Rxc2+ 20.Kxc2 Qxf2+ 21.Rd2 Qc5+ 22.Kb1 Qxa5 23.Rc1 b5 24.Rd7 Bf6 25.Bd6 Qd2 26.Rc2 Qd1+ 27.Rc1 Qd3+ 28.Rc2 Re8 29.Bc6 Re1+ 0-1