Black has a queen and two other pieces—his knight and bishop—all trained on e2, next to White’s king. All that prevents QxN+ from mating is White’s queen. Notice as well that White’s king has no flight squares. As we know, such immobility means that any direct thrust at the king may threaten mate. Black can administer such a check with Nc2. Since White can’t move his king his only legal reply is QxN—but now Black mates on e2. In effect White’s queen again was caught trying to guard two mating squares. When you see the first one, look for another vulnerable square the queen guards from which you might be able to give check, distracting it.