With some regret, White exchanges his Knight for Black's bishop. The White pawn at f5 will
almost certainly be an endgame liability as it will have to be defended with pieces. Black has
decided to castle Queenside. To do this he must defend his own f pawn from the White bishop,
hence the Rook move to g7. Black has a supported passed pawn. The e pawn must start to
march. Black may choose to play d4 and then d5 to force the White knight away. White will
not sit passively watching - there will have to be an attack on the Black king. First things first -
the black-squared white bishop is not vital, and the Black knight at b6 is supporting  the d
pawn's advance.