What does White attack? If you don’t see it, you’re overlooking the king, which attacks Black’s knight. Next question: what prevents KxN? The bishop on e6. White has no way to capture it—but he can attack it with Bc8. A foolish move since it invites Black to play BxB, taking a piece while still protecting the knight? No, because in playing BxB Black would be depositing his bishop onto the back rank, where his king sits. White then would have a check and capture with RxB+; and then after Black moves his king comes KxN, winning a piece for White. The priority of check again. So once White plays Bc8 Black is destined to lose his piece, and Black’s best move thus is Nxf2, allowing his knight to be captured but at least taking a pawn with it as it goes down.