Marshall must have had a high estimate of his powers over Smirka to venture such a dubious experiment. More usual are 5.Ng5, 5.e5, or 5.O-O, all of which offer some protection to the hanging pawn at e4. Though the Bishop pins Black's Knight and prevents the immediate 5....Nxe4, that pin can easily be broken and the threat renewed.
Safer appears to be 5....Be7 when the game Blau-Ammann, Bern 1993, continued 6. Bxf6 Bxf6 7. c3 dxc3 8. bxc3 O-O 9. Na3 Bxc3+ 10. Nd2 Bxa1 11. Qxa1 Re8 12. O-O d6 and Black had a winning material advantage (though he went on to allow a draw). Also of interest is the familiar idea 5....Bb4+!? 6.c3 dxc3 7.bxc3 (7.Nxc3 Bxc3+ 8.bxc3 h6) 7....Be7 to liquidate the d-pawn and deprive White of the best square for his Knight at c3. In either case, Black should be able to maintain his material advantage with careful defense.
White generally plays 6.Bxf6 Qxf6 7.O-O with the idea of attacking in the center with pawn thrusts to c3 and e5. These tactics have been known to succeed against passive defense, and Black's task is not without dangers. For example, on 7....Bc5 8.e5 Black cannot capture the pawn due to the open e-file: 8....Nxe5? 9.Nxe5 Qxe5?? 10.Re1 and wins. In the game Dannenberg-Hennink, Hengelo 1992, White tried instead 8.c3 dxc3 9. Nxc3 d6 10. Nd5 Qd8 11. Re1 O-O 12. h3 Ne7 13. Qb3 Nxd5 14. Bxd5 Bb6?! (14....c6 15.Bc4 b5 seems necessary) 15. e5 dxe5 16. Nxe5 Qf6 17. Re2 c6? 18. Bxf7+ Rxf7 19. Nxf7 Qxf7 20. Re8+ 1-0.
After 6.Bxf6 Qxf6 7.O-O, an interesting
defensive idea worthy of the great Steinitz was tried in the game
DeVisser-Aad, Hoofdgroep 1992, which continued 7....Na5!? 8. Be2!?
(8.e5 Qb6!) 8....c5 9. c3 Nc6 10. e5 Nxe5 11. Nxe5 Qxe5 12. Re1 Be7
13. Bb5 Qf6 14. Qe2 a6 15. Ba4 b5 16. Bc2 d5 17. b4 c4 18. cxd4 Be6
19. Qd2 O-O when Black was doing quite well. Marshall's retreat is
not without precedent, but it seems less dangerous than 6.Bxf6.
After driving back the Bishop, Black must follow through with the straightforward capture 7....Nxe4, when White will remain a pawn down without fully adequate compensation. In the game Jeney-Steinitz, Vienna 1860, Black returned the material to take over the intiative after 8.O-O Nxg3 9. fxg3 d5 10. Bb5 Bc5!? 11. Ne5 Be6! 12. Nxc6 bxc6 13. Bxc6+ Ke7 14. Bxa8 Qxa8 15. Kh1 g4 16. Nd2 Bb6 17. Qe2 h5 18. Qe5 Kd7 19. Rae1 Qg8 20. b4 h4 21. gxh4 Rxh4 22. g3 Rh3 23. Rf6 d3 24. Nf1 dxc2 25. Rf4 c6 26. Qe2 d4 27. Qd3 Bd5+ 28. Kg1 Qg6 29. Qd2 d3+ 30. Ne3 Rxg3+ 31. hxg3 Qh5 32. Rc1 Qh1+ 33. Kf2 Qg2+ 34. Ke1 Qxg3+ 35. Rf2 Bxe3 0-1
After Black passes on 7....Nxe4 by playing 7....d6?, his pawn push on the King's wing suddenly appears to have only weakened his position. Eventually his King will be unable to find a safe haven on any wing.
Smirka must have counted on this pin to hold onto the pawn at d4. But Marshall shows that he is not interested in the return of his pawn: he wants an attack!
Marshall writes that he wanted to prevent Black from castling on the Kingside with this move, encouraging the Black monarch to seek safety on the Queen's wing, where the open c-file might be used to conduct an attack.
A pin is always a useful tactic. Marshall is already piling up his forces along the c-file, and might reinforce the pin on the Knight at c6 with Nd5 and Qa4, trying to break through to the hapless King at c8. It is difficult to come up with a defensive plan for Black.
As C. S. Howell writes, this move is "suicidal." There may have been some hope in the maneuver 19....Bd4+ 20.Kh1 Bc5, though White can continue his assault with 21.b4! After the text move, Marshall breaks through all defenses.
A pretty clearance sacrifice that opens up the long diagonal so that the White Queen can join the attack. There is no hope for Black.
On 20....Nxe5 follows 21.Rxe5! Bxe5 22.Nb6+ winning the Queen.
After 21....Qxc6 22.Ne7+! the Queen again falls in trying to defend the King. There is no way to stop White's attack at this point.
Mate follows 25....Ka8 26.Qc7.
This game offers an excellent illustration of why it is dangerous to push forward wing pawns with h6 and g5 or a6 and b5: as the pawns push forward to attack, they leave weak squares and potential open lines in their wake.