Where does Black have the kernel of a discovery? On the g-file, where his knight masks his rook; friendly pieces next to each other always require a close look. Next Black looks for a target for the rook and sees White’s rook on g2—with the king right behind it. Nf5, threatening the queen to make time for RxR a move later, makes no sense; the rook is guarded, and anyway White can end the threat with RxR+. But when the enemy king is close to your target the natural thought is to try for a substitution, even if it entails a temporary sacrifice. So Black starts with 1. QxR+, and, after the recapture KxQ, now has the makings of a discovered check. He only needs a target that his knight can reach in two moves, and of course he finds it in the queen on h6.