Chess Opening Theory/1. d4/1...Nf6/2. c4/2...g6/3. Nc3/3...d5/4. cxd5/4...Nxd5/5. e4
This move greedily grabs central space. Since Nf6 or Nb6 would leave black without enough pressure in the center, Nb4 misplaces the knight only to create the obvious threat of Qxd4 followed by Nc2, Nxc3 is forced. Although d4 is strengthened, the queenside weakens, and the a1-h8 diagonal loosens up. That is why g6 and Bg7 is played in the Gruenfeld Defence. Black will now play to dismantle white's central ambitions. Usually, Black advances the c-pawn, and then he uses his e-pawn to dismantle white's center. Also, Nb4?? loses to Qa4+ Nc6 d5.
After e4, White has a dangerous center. Black must pressure it or risk a cramped, passive game. The only good move is Nxc3, else play resembles the dubious Marshall defence.