Where is the key weakness in White’s position? Look for alignments of his pieces; by now it should be no trouble to spot the White queen and a2 rook on the same diagonal. An attack by Black with a protected bishop will scare the queen out of the way, leaving White no time to move the rook behind it. White can use the queen to protect the rook, but so long as Black is using a bishop rather than a queen he is happy to make a trade that wins the exchange. He plays a bishop to the diagonal with Be6, the protection being furnished this time by a pawn, and wins the exchange next move.