The Black and White queens attack each other, and both are protected by their knights; Black can't take White's knight, so the position might seem to be deadlocked—but not if you obey the practice of examining your checks. For then you see Black's Nf3+. If White deals with the check by capturing the knight with his rook or g2 pawn, consider the resulting board: now Black's bishop has a clear line to the knight on d4, and Bxd4+ is another check that continues to keep White too busy to play QxQ. White extinguishes the check with c3xB, and then loses his loosened queen to QxQ. In effect Black played a discovered attack against the guard of White's queen.

In reply to Black’s original check Nf3, White also has the better option of moving his king to h1. That ruins Black's follow up of Bxd4 because the move no longer gives check; thus White can instead reply QxQ+. But White's Kh1 allows Black a different follow up instead: NxN, also removing the guard. True, it doesn’t give check; but now the piece that removes the guard also now protects its queen. Thus when White now plays QxQ+, Black can respond with NxQ. He has won a piece and moved his knight out of danger.