The sight of Black’s king and knight aligned on the same diagonal should be striking. Until now it might not have seemed meaningful because no pin is possible; the positions of the king and knight would have to be reversed for that. But this ordering nevertheless makes possible a skewer. Examine any checks White can give and see that there are two: Bh3+ and Bc4+. Both require the king to move; but when you imagine it moving in reply to Bc4+ you see a line left clear to the loose knight on g8—which you take.