Good, you should show the entire varation : there are two tactical motifs here : discovered double threat and elimination of the defending piece : 1. Ng5 Bxg5 2. Bxc6 Bxc1 (or 2... Rb8? 3. Bxg5! Qxg5 4. Bxd7 winning a piece) 3. Bxa8 Qxa8 4. Rxc1 winning the exchange.