The Outpost

The diagram shows that White has the center and the d-file. In other respects positions are equal. White with the move will attempt operations on the d-file. This presents difficulties since the protected Black pawn at d6 represents a “granite block”. The key move in this position is 1.Nd5 and the knight placed here we call the outpost. By outpost we mean a piece, usually a knight, established on an open file in enemy territory and protected by a pawn. The knight will exercise a disturbing influence due to his radius of attack and will cause the opponent to weaken their position in the d-file, in order to drive him away, by …c6.

• An advanced outpost forms a base for new attacks
• An outpost provokes a weakening of the enemy’s position in the file in question.

from My System by Nimzowitch