plang: 7 b3 with the idea of exchanging dark-squared bishops is a system attributed to Botvinnik.
Botvinnik after 13 e3:
"Had the black pawn been at f7, the game would have been completely equal. Now, however, despite the following simplification, White retains a slight positional advantage, for the reason that this pawn is at f5 - the weakness of the e5 square may tell."
Bronstein after 30..Nf6:
" During the game I was not so confident about the harmlessness of the ending after the simple 31 Rxc7+..Rxc7 32 Rxc7+..Qxc7 33 Qa6..Qb8. Now I see (as Botvinnik saw then) that White has no realistic way of breaking through. But at the time my character did not allow me to await the future passively, and as a result - my "plat du jour"."
In mutual time trouble both players missed, after 32 gxf?!, the response 32..Rxc2 33 Rxc2..Nh5 34 Nd3..Rxc2 35 Qxc2..Nxf4+ 36 Nxf4..Qxf4 with an even ending. Bronstein, in his calculations, had overlooked 35 Qg4 leading to a winning knight ending. After the alternative 40..Nc1 41 Nxd5..Nxb3 42 Ke3 the Black knight would have been trapped.
Botvinnik after 43 Ne5:
"White's plan is straightforward: post his knight at d3, and then begin making use of his extra pawn on the queenside. It is important, only, that in so doing he should not allow the enemy king to approach his f and h pawns."
The alternative 53 Nc5+..Nxc5 54 dxc..Kd7 55 Kd3..Kc6 56 Kd4..h4! would have given Black too much play.