Where does White have the makings of a discovered attack? On the third rank, where the bishop masks the queen. Well, in practice you probably would not think quite like that, since the queen so plainly has no target on its rank if the bishop moves out of the way. The real key to seeing what can be done here is to examine Black’s king and its vulnerabilities. It is stuck in the corner; White’s bishop on b3 covers g8. And the file leading to the king is open. The king’s lack of pawn cover and constricted movement cry out for an attack along the h-file; if White could get one of his heavy pieces (the queen or a rook) onto h3, it would be mate. Therein lies the winning idea, for White’s queen already is aimed at h3. If the bishop moves out of the way, White’s threat against that square thus is a mate threat, and requires a response from Black just as a check would. So now all White needs is a dark-squared target that his bishop can reach in two moves. The queen is the target of choice; Bxb6 wins it after Black plays Nf8 (preparing to fend off the mating threat with his knight).