Chess Opening Theory/1. d4/1...d5/2. c4/2...c6/3. Nc3/3...dxc4/4.e3/4...b5/5.Nxb5/5...cxb5/6.Qf3
This move tries to regain the material, hopefully with interest. Now, we come to a cross, where two moves lead to a clear black advantage. The first idea is to cut Black's losses to a piece white maintaining the queenside. This can be achieved simply by Nc6, when after Qxc6+ Bd7 Black's queenside holds. The other, more dramatic plan is to imprison white's queen on a7 at the cost of a rook. This is achieved by Qc7! Qxa8 (Now, cut off the long diagonal so the Queen can't escape with her capture) Bb7! Qxa7 (Now, cut off c5 and a3 and the queen will get trapped) e5! (e6 is also good). Now, the center is opening, and Nc6 is threatened. White will have to lose material in order to save his queen.
Black can also play the extremely interesting Na6 Qxa8 Nb4, with an attack for the lost exchange. However, it is not clearly better for Black, and computer analysis proved that it is only a draw after Rb1 Nc2+ Kd2 Nf6 Qxa7 (Avoiding Kxc2?? Bf5+) a3! (Stopping Bb4+) Bf5 Qc6+ Qd7 Qa8+! (If Qxd7+?! Kxd7, black has the better ending since the rook will be won back due to the Bf5/Nc2 discovery for example: Let's say that white passed this move Nxd4 Ra1?? (White had to give back the material) Nb3+, 0-1) Qd8 Qc6+ Qd7, =.