Black has a single capture of a piece available: QxN. Obviously it doesn’t work because White’s knight is guarded by his queen. But at least you see that White’s queen is working. Is it protecting anything else? Yes; the queen also is the sole guardian of the bishop on b2. You have nothing you can use to take the bishop, but you inspect your knight moves and see that Nd3 attacks it—and also forks the king. How might White reply to this move?

(a) If he plays QxN he loses his queen to Black’s rook, so instead he no doubt will move his king.

(b) If White moves his king to f1, his queen is left overworked: it is the only guard of the knight on f3 and the bishop on b2, both of which now are attacked by Black pieces. Thus Black plays NxB; and if White recaptures QxN, Black has QxN.

(c) If White instead responds to Nd3 by moving his king to d1, be alert: he has just walked into the kernel of a discovery on the d-file; now Black plays Nf4, discovering check and taking White’s queen next move.

It is common enough for an enemy piece to guard two others, and for you to attack only one of them—as we find in the initial position here. Throwing an attacker at the other enemy piece (i.e., moving a piece into position to attack White’s bishop) usually does not create a crisis for your opponent, because now it's his turn and he can either move one of the two attacked pieces or give it another guard. An exception to this logic arises, however, if you can add an attack to the second piece with a double threat—as Black did here with his knight fork. Then there is no time for White to move one of the attacked pieces. He has to spend his time saving his king, and after he does that his queen is left overworked.