Let's see if we can find some reasoning behind trading towards an endgame.
For reasons of convenience I start with the most common trades. That are the trades of pieces of equal value.
B x B
B x K
K x B
K x K
R x R
Q x Q
Underlying pawnending is a win.
Since the value of the pieces is equal, there must be something else that determines if a trade is beneficial. That beneficiallity is based on the underlying pawn ending. You have to ask yourself first "is this position won if there were no pieces on the board". I said a few things about that here and here some time ago. If the pawnending is won, you have a sustainable advantage and you can start to trade pieces until there are no pieces left.
Underlying pawnending is a draw.
If the pawnending isn't won but a draw, there are other matters that play a role to determine if a trade is beneficial:
- activity of the pieces.
- does the trade alter the pawnstructure.
In the ending the piece activity is two fold: attacking, can a piece help to clear the road to promotion, and defensive, can your piece restrain or block an enemy pawn to prevent it's promotion. This piece activity is mainly influenced by the pawnstructure. Your own pawns block your own pieces, and squares that are under attack by hostile pawns are usually forbidden. The idea is to trade your pieces in such way that you keep the active ones while your opponent is saddled with the passive ones.
But before you can decide on trading you have to ask yourself wether the pawnstructure is fixed or volatile. If the position is very volatile because the pawns are not restrained or blocked, the piece activity can easily change by just moving pawns.
If a piece is defended by a pawn, the trade of equal valued pieces alters the pawnstructure. This can effect the outcome of the underlying pawnending and/or the piece activity. This should tell you if the trade is beneficial or not.
It is all so logical that I wonder why I haven't thought of this before.