Figure[Black to move]

By moving to e2 Black’s knight would fork White’s king and queen. But the needed square is under protection—twice, from White’s knight and rook. Each of the guardians is attacked only by Black’s queen. So imagine taking out one of them, and then imagine taking out the other. If QxN, then White replies f2xQ (or BxQ) and the White rook at e4 still guards the needed square. But if Black begins with QxR, then White replies NxQ and now e2 is available for Black’s knight. Once more, an exchange of one of the two guardians effectively gets rid of both of them, as guardian #2 has to recapture the piece that captured guardian #1.