Again we consider how an idea seen in prior chapters can apply to the knight: sometimes the piece that unmasks a discovered check can, instead of hunting for material, add more to the pressure on the king or otherwise take part in the creation of a mating net.
The position to the left shows how it's done in simplest form. See the kernel of the discovery for Black on the e-file; see that the piece to be unmasked, the queen, is aimed at White’s king. Should the knight go after material, for example by jumping to g4 and then taking the White bishop on f6 on the next move? No, Black should play the knight to f3, delivering checkmate. From f3 the knight adds a second check, which means the king has to move; and the knight seals off d2, the king’s only flight square.
Notice that from the outset Black is threatened with BxQ. This is another reason why a simple discovered check like Ng4 would fail; White would just take Black’s queen. The only discovered checks that will work here are double checks, because they require your opponent to move his king.