DanQuigley: On pages 242-244 of BCM are George A. Thomas's excellent notes to the game. I found them to be so enlightening that I provide them here in full (they are out of copyright):
GAME No. 4,942.
Played in the Weston-super-Mare tournament. Notes by G.A.T.
Queen's Pawn Opening.
WHITE: B. Kostich
BLACK: G. Maroczy
1.d4 Nf6 2.Nf3 e6 3.c4 c5 4.d5
White could of course keep to the normal lines of the QP Opening by 4.e3.
Practically an unknown move at this stage. It is said, however, to have been analysed some years ago in a continental magazine by its inventor, Blumenfeld. Though clearly an innovation of importance, the analysis does not seem to have attracted much attention. Curiously enough after the idea had lain dormant so long without being tested in an important game, Aljechin adopted this move against Tarrasch in the Posteyn tournament, almost simultaneously with Maroczy's exploitation of it in this game. Black won that game also, though the games diverged from Black's 5th move.
The alternative would be 5.e3. The text-move allows Black to obtain a very powerful centre at the cost of a Pawn.
5...Bb7 6.dxe6 fxe6 7.Nc3 d5 8.e3 Bd6 9.Be2 O-O 10.O-O Qe7
Black has obtained a splendid position in exchange for his Pawn. Here, however, he should apparently play 10...Nbd7 first, following with ...Qe7. The transposition might have had a distinctly adverse effect on his game.
For now White might have attacked the centre by 11.e4, threatening e5. If then 11...dxe4 12.Ng5 h6 13.Nh3, and though White has returned the Pawn he has broken Black's centre; or if 11...Nbd7 12.exd5 exd5 13.Bg5, and the menace of the Black centre has been considerably lessened. Had Black played 10...Nbd7 instead of 10...Qe7, White would not have had this resource; as in that case 11.e4 would be answered by ...d4, with advantage.
Played with the intention of inviting …c4, and so opening the square d4 for his Knight. Against this must be reckoned the loss in time, and the fact that the Black Knight comes in powerfully at c5; but it is not easy to suggest a more promising line.
11...c4 13.Be2 Nc5 14.Nd4 Nfe4 15.Nxe4 Nxe4 16.f4
A choice of evils. This closes some of Black's lines of attack, but enables him to bring a deadly attack to bear on the e-pawn. The Black Knight cannot be driven away by 16.f3 because of 16...Qh4 in reply.
Of course 17.fxe5 Qxe5 would be fatal; for if then 18.Nf3 Rxf3; or 18.g3 Nxg3.
17...Bxc6 18.bxc6 exf4
If 19.exf4, then 19...Bc5+ 20.Kh1 Qh4 threatening ...Ng3 mate.
19.Bf3 Bc5 20.b4
A desperate attempt at counter attack.
20...Bxb4 21.Rb1 Bc5 22.Rb7
Black can ignore this move. Nevertheless, White's only hope lies in raising a counter-attack.
Of course his Queen can not be taken as 23...e2+, would be immediately fatal.
Preferring to maintain the attack, rather than to play for a winning end-game, which he could now obtain by 23...Nc3 24.Rxe7 (best), Nxe2+ 25.Bxe2 Bxe7.
24.Bb2 c3 25.Bxe4 cxb2 26.Bf3 Qxc6 27.Rxb2 Bb6 28.g3 Rf5 29.Kg2 Raf8 30.Rc2 Qe6 31.Qd3 Qf7 32.Qe2 h5
Not really necessary. But White can do nothing; and Black can afford to take his time.
33.h4 d4 34.Qc4 d3 0-1
A very finely played game by the winner.