McMurphy, who has hitherto been motivated only by what will help himself, begins to feel the camaraderie of the group. His loyalty to the others is what causes him to subjugate himself to the goals of proving that he cannot be made a prisoner. And, so he breaks through the window to the other side.
McMurphy's breaking of the window was a symbolic gesture of defiance against the suppression of the individual by society. It represents McMurphy's changed attitude in the play.
Intially McMurphy's actions of defiance were for selfish reasons. They were simply a means of wasting time until his release. However in Part 2, McMurphy has an epiphany. He realises the fact that the other men rely on him and are caged/emasculated by a repressive society symbolised by Nurse Ratchet and the ward. This realisation is sparked the knowledge that all the acutes are in hospital on a voluntary basis besides himself and Scanlon. From this point on McMurphy's becomes a selfless individual. By breaking the window, McMurphy becomes a martyr for the men, reminding them of the intrigue of the ward, and challenging them to regain there identity.
The effect of this on Chief Bromden is to return him to his erstwhile size and power. This gives him the courage to leave the hospital and confront the repressive society of which the ward was a microcosm. This change is symbolised by a change in his narration. He no longer uses the 'fog machine' as a means of hiding. In addition, he ceases to be an introvert towards the other patients, dropping the facade of being 'deaf and dumb'.