[Footnote:  Following are the two main variations which have been adopted in the practice of the Masters: 

A. (5) o-o, B-e7; (6) R-e1 (protecting the Pawn e4 so that Black has to defend himself against Bxc6 and Ktxe5), P-b5; (7) B-b3, P-d6; (8) P-c3 (preserving the Bishop against the exchange threatened with Kt-a5), Kt-a5; (9) B-c2, P-c5; (10) P-d4, Q-c7.  Now the Pawn e5 is sufficiently protected and Black threatens to exert pressure on d4 by playing B-g4 and Kt-c6.  It might seem that White can ignore this threat and as soon as d4 attacked either exchange the Pawn on e5 or advance him to d5.  However, the former maneuver would open the d-file for Black’s Rooks and the latter would not be very good either before Black has castled; for the advance of the d-pawn closes the center-files permanently for the Rooks so that Black’s King is safe in the center while White may have to face an attack on the King’s wing which Black might initiate by storming with the h and g Pawns.

For all these reasons White is justified in making the waiting move (11) P-h3, which safeguards the square d4 against the indirect attack B-g4 and at the same time prepares an attack along the lines of the one discussed in connection with Diagram 52.  After (11) ..., Kt-c6; (12) B-e3, o-o; White can safely advance the Queen’s Pawn and then launch the attack referred to above.

B. The second variation arising from the position of Diagram 62 is (5) o-o, Ktxe4.  It seems rather dangerous for Black to take the Pawn as long as his King is in the e-file which White can occupy with his Rook; but after (6) R-e1, Kt-c5; (7) Ktxe5, Ktxe5; (8) Rxe5 +, Kt-e6, White has not much attack and for this reason the opening is generally treated in a different way.  As the strongest move is considered (6) P-d4.  Black can hardly take this Pawn as R-e1 and Ktxd4 would follow threatening to win a piece; but he may try to maintain his Knight on the dominating square e4 by (6) ..., P-b5; (7) B-b3, P-d5.  After (8) Pxe5, B-e6; (9) P-c3 (again providing a retreat for the Bishop), B-e7; (10) Ktb1-d2, Black’s best continuation is probably Ktxd2; (11) Qxd2 (not Bxd2 on account of B-g4); Kt-a5 or o-o.  The maneuver (10) ..., Kt-c5; (11) B-c2, P-d4, which was tried in several recent tournaments, is of doubtful value as Black loses control of the important center-square e4.  White can use this square for an attack on the weak point c6 as follows:  (12) Kt-e4, Pxc3; (13) Ktxc5, Bxc5; (14) B-e4, Q-d7; (15) Q-c2, B-d5 (in order to answer R-d1 with Bxe4); (16) B-g5, and Black does not seem to have a satisfactory defense.  For instance (16) ..., B-b6; (17) Ra1-d1, Bxe4; (18) Qxe4, Q-e6; (19) R-d6 or (16) ..., Bxe4; (17) Qxe4, Pxb2; (18) Ra1-d1, P-b1 (Q); (19) Rxb1 followed by R-c1.]

(5) ...  P-b5

The simplest and probably best way to defend the Pawn e5 against the threat Bxc6 and Ktxe5 is P-d6 without P-b5.

(6) B-b3 P-d6

The alternative was B-c5.  White could then not have captured the Pawn e5 by (7) B-d5, o-o; (8) Bxc6, Pxc6; (9) Ktxe5 on account of Q-d4, threatening mate on f2 and attacking the Knight.