White has just played h2-h3, threatening to take Black’s bishop. What is Black to do? He could retreat the bishop, but around here we don't play defense before looking at offense. White’s position looks impregnable but has a subtle weakness: follow the lines from White’s king and you see that the pawn on f2 is pinned. That means it can't guard anything; in particular it means White’s knight on g3 is loose. Instead of retreating his bishop, Black thus can play NxN and let his g4 bishop get taken. Why is this better than retreat? Because after NxN Black’s knight attacks White’s rook. If White moves the rook, Black moves his bishop and has won a piece cleanly. If White takes Black’s bishop, Black pays NxR and wins the exchange after White recaptures. It’s another study in how the pawns in front of a king usefully can be pinned, leaving pieces on the third rank open to capture.