Tactics: Insisting on a Gruenfeld

Consider the following...    

a) exd4  
b) Rab1  
c) e4  
d) Be4

20.exd4? leaves White hanging at least a pawn to Bxd4, and even more to 20... Qxd4  21.Qb3 Qg4 when White is hanging the Bg5, the exchange on a1, and the Pb4.

20.Rab1? defends the Pb4 and gets the Ra1 off the line of the Bg7, but doesn't save a pawn or the exchange, since Black has a choice between the simple: 20... Nc3 +/- or the more complicated: 20...dxe3  21.fxe3 Nxe3  22.Qa5 Ng4+!  23.Qxb6 axb6 and White's Bs aren't nearly enough compensation. 

20.e4! is the best of a bad bunch of choices.  

20... Qxb4 21.Qxb4 Nxb4 22.Bc4!  Black is up a pawn, but the B-pair makes sure he'll have to play well to convert; e.g.:

22...a5 23.a3 Nc6 24.Bxe6!?,
22... Rfc8  23.Rab1 Rab8 24.Bf4 e5 25.Bd2 Bf8 26.f4

 In the game, White tried the most active-looking move: 20.Be4? dxe3  21.Bxe3 Nxe3  22.Bxa8 Bxa1  23.Rxa1 Rxa8?  This keeps a huge advantage, but it's not the best.

Better is 23...Ng4! Like the game line, Black wins a pawn (by leaving White hanging on f2 and a8); but it keeps the N on the board, which gives Black a winning attack: 24.Bf3 Qxf2+  25.Kh1 Qd4 -+ threatens both smothered mate and winning the Ra1.

24.fxe3 Qxe3+  25. Kh1 Rd8  26.h3 Qf2

Black is up a pawn with a safer King and the more active major pieces, and went on to win:

27.a3 Rd2  28.Qc6 Qd4  29.Rf1 Qd7  30.Qc4 Qd5  31.Qc8+ Kg7  32.Qc3+ e5  33.Qg3 Rd3!  34.Qg5 Rxh3+  35.Kg1 Qd4+ 36.Rf2 Qd1+!  37.Rf1 Rh1+  38.Kxh1 Qxf1+  39.Kh2 Qf4+  40.Qxf4 exf4  41.b5 Kf6  42.a4 Ke6  43.a5 Kd6  44.Kh3 f5  45.Kh2 Kc5  46.b6 axb6  47.a6 Kc6  0-1