Why is a passed pawn usually considered a great asset? By definition, a pawn is passed when it can advance to promotion without encountering any opposing pawns in its path. Possession of a passed pawn and the ability to exploit its potential is a strategic element that can often determine the result of the game. A passed pawn is sometimes colloquially called a passer.
The passed pawn may prove significant in the middlegame, gaining space and tying down the opponent’s pieces, but its true strength comes to the fore in the endgame. In practically all types of endgames, possession of a passed pawn is considered a decisive advantage, particularly when the remaining material is scant. Even if it proves impossible to promote the pawn, its mere presence is enough to restrict the enemy pieces, force material gain or simply maintain the initiative. Consequently, this strategic element greatly influences — and is influenced by — the matter of piece exchanges.
A passed pawn must be blockaded, so as to have its power restrained as much as possible. The minor pieces (knight or bishop) are ideal for this purpose, as they can rarely be forced to retreat by enemy action. On the other hand, the major pieces (queen and rook) find it difficult to achieve a stable blockade as is easy to harass them, while one must also consider that, for such valuable pieces, dealing with a mere pawn cannot be an efficient form of employment. Taking the above into account, it becomes clear that the side with the passed pawn should seek to
exchange minor pieces and retain the major ones; the opposite applies to the defending side.
A factor of crucial importance is the ability of the defending king (i.e., the one facing the passed pawn) to participate in the proceedings. After exchanging the major pieces (and especially the queens), the king can approach the passed pawn and blockade it (or generally stop its advance), thus freeing the other pieces of its army for other duties. A passed pawn that has been securely blockaded and efficiently neutralized may become a weakness and then this very important element may even lead to the loss of the game.
In general, the side possessing a passed pawn has clearer plans and aims. The other side usually seeks ways to blockade it or, if this proves impossible, obtain counterplay on another part of the board. One good option is harassment of the opponent’s king. In practice this option often proves very effective, but unfortunately it is not always available! The most fundamental rule of exploiting this strategic element is: passed pawns must be pushed!