The more standard approach is 7....Bd7 8.Bxc6 bxc6 9.O-O Bc5, when White gets a good game with 10.f3 Ng5 11.Be3.
There is some debate over whether White might be able to accept Black's supposed "pseudo-gambit" here, since 8.Nxc6!? Bxf2+ 9.Kf1 Qh4 10.Nd4+ c6 11.Nf3 Ng3+ 12.Kxf2 Ne4+ 13.Ke3 Qf2+ 14.Kd3 Bf5 (Keres's old analysis ends here with Black's advantage) 15.Nd4! Bg6 16.Qe1! and, according to Dlugy, the position may favor White. For example, if 16....cxb5? 17.Qxf2 Nxf2+ 18.Ke2 Nxh1 19.Bf4. But Tim Harding, following Kujif, recommends the line that Adolf Anderssen preferred back in 1861: simply 7....Bc5!? 8.Nxc6!? Bxf2+ 9.Kf1 bxc6! 10.Bxc6+ Kf8 and "it is hard for White then to contain Black's rampaging pieces" he writes. For example, 11.c4 Ba6 12.Na3 Bc5! 13.Bxa8 Qh4! But Marks Morss notes that "Schlecher in his Handbuch claims that Kujif's idea is refuted by 11.Qd3," and he quotes Schlechter's extensive analysis. So who do we believe? Those interested in these fascinating byways are encouraged to read Morss's extensive analysis of his postal game with Clark, available online:
No doubt the safest course of action, though, is 8.Be3.
Tim Harding recommends that White play simply 10.Nd2! here to avoid Black's tricks with 10....Qe7 as in the game. Then if 10....Nxd2 11.Qxd2 Qe7 12.Nb3! Bb6 13.Qc3! gives White good play on the dark squares. Less clear is 10.f3!? Qh4+ 11.g3 Nxg3 12.Bf2 when Black should not play 12....Qh3 13.Bxg3 Qg2 14.Rf1 Bh3 15.Qd3 Bxd4 16.Nd2! Bb6 17.O-O-O with a strong game for White, but, as C. S. Howell points out, Black can play for a draw with 12....Qh6! 13.Bxg3 Bxd4 14.Qxd4 Qc1+, etc.
Best here is 11.Re1 O-O 12.f3 Ng5 13.f4 Ne6 14.c3 with about equal play or 11.f3!? Nd6! (Better than 11....Ng5 12.f4 Ne4 13.Nd2 +=) 12.Bf2 Nf5 13.c3 =. White's move seems to risk losing the e-pawn, though Black should not risk taking it immediately because 11....Qxe5?! is met by 12.Nxc6! Qxb2 13.Bxc5 Bxc6 (not 13....Qxa1?! and White gets a terrific attack with 14.Nd2 Qf6 15.Nd4 Qf4 16.Rd1!) 14.Nd2 (14.f3? Bb5!) 14....Qxc2 15.Bb4 O-O-O (the King is too exposed in the center, though he is not much safer on the wing) 16.Rfc1 Qa4 17.Nxe4 dxe4 18.Rab1 and White's initiative gives him more than sufficient compensation for his sacrificed material.
Black should not grab the b-pawn immediately since 12....Rxb2? is met by 13.Nxe4 dxe4 14.e6! (a thematic move in these positions) 14....fxe6 15.Qh5+ g6 16.Qe5 with a winning attack. But notice how Marshall has patiently accumulated his threats to the point where White must lost material.
If White is going to sacrifice his center pawn, he does best to try 13.Nxe4 dxe4 14.e6!? Bxd4 (not 14....fxe6 15.Nb3 with a great game due to Black's many weak pawns) 15.exd7! (White remains down a pawn after 15.Bxd4, though the Bishops of opposite color and Black's weak pawns give him compensation) 15....Qxd7 (15....Rxb2 16.Rad1) 16.b3 and White wins back his pawn with complete equality.
Black has simply won a pawn and White's compensation is difficult to spot.
White risks further weakening of his position. Best here was 17.Bf4 with the idea of eliminating Black's better Bishop with a plan of attacking Black's weaker pawns.
Now he is up two pawns. White seems to have been overly careless of material.
Marshall saw that if 22....Rb8 then 23.gxf5 Bxf5 (or 23....Qxf5 24.Bd4 Rf7 25.Rg1) 24.Rg1 gives White a slight initiative due to his pressure on g7 and on the dark squares generally. Therefore he returns some of his material in order to maintain the initiative. Marshall valued the initiative above all else and so was willing to give up the exchange for a piece and several pawns so long as he did not have to go on the defensive.
If 24.fxg4?! Rxf2 25.Bxf2 Bxg4 with a powerful attack for Black on the white squares. Likely best was 24.Rg2, hoping to organize a defense. Now Marshall busts open the White castle position for a winning attack.
White must try 25.Rg2 Bh3 26.Rxg3! Bxg3 27.Rg1! with at least some hope of fending off immediate disaster.
And now Black gets the exchange back with this skewer attack.
Black should first deny White any counterplay by grabbing the e-file with 28....Re8! Now White finally organizes his counterattack, forcing a relatively difficult ending.
Black forces off the Queens to put an end to White's chances of attack on the dark squares. But the Bishops of opposite color ending must have given Bigelow some hopes of a draw.
A faster road to certain victory was afforded by 40....d2 41.Rd6 d1=Q 42.Rxd1 Rxd1 and Black's extra Rook should compel White's resignation. Black now threatens to queen with 41....Rg1+ and 42....c8=Q. But White can stop him from queening on the dark square thanks to the opposite colored Bishops.
After 44.Bxc1, White would have achieved his Bishops of opposite color ending and might rightly have hoped for some drawing prospects against most players. One must remember, however, that this was Marshall's last game in the tournament and that he was guaranteed first prize no matter what the outcome. Therefore he was happy to have achieved this ending position, since his connected passed pawns should guarantee the win with careful play while the Bishops of opposite color presented no risk of losing should he tire and make a mistake. Though Bigelow's blunder likely pleased Marshall, it unfortunately deprived us of the grandmaster's demonstration of how these endings are won.