Black’s knight is on the mighty d4 square—mighty in part because from there the knight can jump to e2, checking White’s king and forcing it into the corner where it may be subject to Anastasia’s mate. Black considers whether he can open the h-file and put a heavy piece on it to complete the pattern. White’s pawn on h3 is the troublemaker; it protects the knight at g4, however, so Black can play QxN to draw the pawn out of the way. But now he must worry about how to get a rook to the h-file: after White’s recapture his pawn left on g4 would guard h5 against occupation by one rook and block the path to the h-file of the other one. Yet those two points taken together suggest a solution for Black. He goes ahead with Rh5+ (always working with checks where he can), inviting g4xR—this time clearing a pawn by launching a threat from a square it protects. Now the way is clear for Black’s other rook to go to h4 and mate. To summarize the sequence: 1. …Ne2+; 2. Kh1, QxN; 3. h3xQ, Rh5+; 4. g4xR, Rh4#.

This is one of those ideas that is important not because Black is likely to carry it out but because the threat of it will force White to cough up material. For example, as soon as Black’s knight moves to e2, White can take it with his queen—but then the queen is lost to RxQ. Or White can permit Ne2 but then respond to Black's second move—QxN—by letting go of the knight and keeping the h-file clogged with a pawn. Thus White could reply to 2. ...QxN with 3. Qxd6; or White might play the pawn fork 3. f2-f3. In response to this last threat, Black would have 3. ...Ng3+ (looking first for his checks, since they hold the initiative and prevent White from carrying out the fork), 4. Kg1, NxRf1. However White recaptures, Black has made gains and threatens NxQ if White tries to make good on the pawn fork. White can move his queen away, yet Black has good replies then, too—but now we're getting into complications beyond the scope of this little study. The general point is that Black gains the upper hand one way or another by putting the mating sequence into motion, even if he doesn't actually get to execute it.