White aims to restrict the black knight by g2-g4, and for this idea to work he must first swap a few pieces.
The key move, which leaves the black knight badly placed for the rest of the game.
Thanks to his strong passed pawn and the weakness on d4, Black is clearly better despite being the exchange down. The game continued:
8.Rb1 Rb8 9.h4 Rb7 10.Kh2 Kxd6 11.g4 Nc3 12.Ra1?! Nb5! 13.Rd1 Ra7 14.h5 g5! 15.Kg2 Ra2 16.Be2 Rxc2
Preparing h2-h4 in order to prevent the plan of ...Nh7 and ...Bg5 with a favourable bishop exchange.