Though this forces White to exchange Bishop for Knight, weakening White's coverage of d2 and strengthening Black's pawns slightly, it appears also to be a crucial loss of time that allows White to activate his pieces while it also puts another pawn on a dark square - weakening the light squares on the kingside. Marshall himself chose 14....Bb4! in his game against Forsberg as Black, though Forsberg could have kept the advantage with 15.Ne2! instead of 15.Bxf6?!
The Rook puts pressure along the e-file and prepares to go to e3 to defend the Knight (compare the earlier game Torre-Marshall where White did not have time to play Re3 due to the weakness of his back rank). Once the Rook reaches the third rank it can attack all of Black's weak pawns.
Another Black pawn is forced onto a dark square, weakening the light squares, as we shall see. Worth considering, though, is 16....Bb4!? 17.Rxe6 Bxc3 18.bxc3 Rd2 19.Rxf6 Rxc2 20.Rf3 with a clear edge for White, who has an extra passed pawn, but Black has some play after 20....Re8!? with the idea of R8e2.
Black does not want to surrender his hold on the seventh rank. He threatens 20....Rd1+, and if 20.Nxd2?! Bxd2 with a slight edge for Black. But in the long-run, this move does not bring about equality due to the weakness on the kingside. It is difficult to say, though, what Black might do instead....
A strong multi-purpose move, which creates luft for White's King while helping to dominate the white squares on the kingside. Once White fights back Black's attack he will have a clear plan of attack on Black's position.
Black changes plans since nothing comes from 20....Rd1+ 21.Rxd1 Rxd1+ 22.Kg2, when he suddenly has even fewer defensive resources. White is also prevented from playing 21.Ng3 Rxg4 or 21.Nxf6 Rg6! 22.Rf3 Be7 =. But after White's next move, Black loses material.
Protecting the pawn while breaking up the fork after Nxd2. White now must win a pawn due to the fork at d2 and f6. The following complications are difficult to foresee completely, but White remains always one attack ahead, which is enough to understand that White will come out on top.
The dust has cleared, and White remains a solid pawn to the good - and a passed f-pawn at that. There is really no hope for Bigelow against a player of Marshall's technical prowess.
Black is forced to exchange Rooks or part with another pawn. The resulting King and Pawn ending, though, is completely lost due to White's passed f-pawn.
Of course not 37.Rxc5?? b7! and Black queens.
White has too many technical problems to solve if he instead tries to use the passed pawn as a distraction with 45.Kc5? Kf6 46.Kxb5 Kxf5 47.Kc5! Kg4 48.b5 Kxg3 49.b6 h4 50.b7 h3 51.b8=Q+, and despite queening with check the win is still quite difficult.
Meanwhile, after 45.Ke5, White has the opposition and Black's King must give way: 45....Kf7 46.f6 Kf8!? 47.Ke6!? (White can also win the h-pawn with the more thematic opposition-keeping move 47.Kf4!, but this is cuter) 47....Ke8 48.f7+ Kf8 49.Kf6! (this would be stalemate, of course, if Black could not move his pawn at h5) 49....h4 50.g4! (50.gxh4?? is stalemate) 50....h3 51.g5 h2 52.g6 h1=Q 53.g7#!