Swapping the more active (to say the least) of Black's minor pieces. This also helps to improve his own bishop,

Finally Black is in zugzwang. There followed:

White soon won.

Imprisoning three black pieces on a6, a8 and b8.

Overprotecting the crucial d6-pawn.

Very clever. It is worth a pawn to swap off Black's active pieces, so that he is left with only bad ones.

5...Qxe4? 6.Bf3 +-

Despite his minus pawn, White is winning.

Opening a new front on the kingside.