"By transposition of moves (Petrosian and Pachman) arrive at a kind of closed variation of the Sicilian, a complicated opening because both sides have various possibilites at their disposal. This wide choice of moves brings with it many mistakes which are not realized until it is too late. The Czech GM (Pachman) unwisely moves his QP, giving White's Knights the chance of circling round the black Queen which is drawn over to the Q-side. Petrosian profits from this by the "cou de grace" on the K-side with a brilliant Queen sacrifice"
SOURCE:"Tigran Petrosian - World Champion" 1965 by Count A.O'Kelly de Galway pp 85-7 (all unattributed analysis/commentary)