This week’s example sees Petrosian make a surprising offer to exchange off his good knight for Black’s bad bishop. However, two factors make this the right decision:
1. The “bad bishop” on e6 is actually doing a very solid job of defending the weak pawn on d5 (bad bishops are frequently good defenders of weak pawns, as Mark Dvoretsky has pointed out)
2. After the exchange, the remaining white bishop is much stronger than the black knight. This is another example of the rule which governs all exchanges: look at what stays on the board, not what comes off.