Ulhumbrus: According to Petrosian the move 29...Bf5 increases Black's pressure.
If the reason for this is that after 30 Bxf5 Qxf5 Black's Queen occupies the b1-h7 diagonal, this suggests refraining from making that concession by 30 Qc2.
However against the move 30 Qc2 Petrosian may have had prepared the following combination: 30 Qc2 Qxe3!! displaces the f2 pawn which defends the fork ...Nxe3 forking White's King on f1 and Queen on c2.
On 31 fxe3 Nxe3+ 32 Ke2 Nxc2 makes a X-ray capture, Black's Queen's Bishop attacking White's Queen right through White's Kiong's Bishop.
On 33 Bxc2 Bxc2 attacks the N on b3 which defends the White Rook on a1.
On 34 Nd2 the White Rook on a1 is undefended and so the White N on a2 is pinned to White's Rook on a1 and so cannot defend the b4 pawn and so allows 34...Bxb4
On 31 Bxf5 Qxb3! both draws White's Queen on to the square b3 which can be forked by a N on d2 and removes the N on b3 which covers the d2 square.
On 32 Qxb3 Nxd2+ 33 Ke1 Nxb3 displaces the White Rook from its defence of the N on a2 and on 34 Rb1 Rxa2 35 Rxb3 Black has regained the piece and with interest.
This brings to mind at least one game by Nimzowitsch where some attempt on his opponent's part to avoid an unwelcome move would fall into a combination.