How does Rodger get away with elbowing Jack out of the way right at the end of chapter 11 when Jack is trying to intimidate Sam'n'Eric?

Answer 1

Roger doesn't get away with elbowing Jack out of the way at all mainly because he doesn't make any contact with him at all. To quote directly from the book... Roger edged past the Chief, only just avoiding pushing him with his shoulder.

Answer 2

Rodger gets away with elbowing Jack because he is already under Jack's influence and has been conditioned to be aggressive and violent. Jack's authority and power over the boys allow Rodger to act without fear of consequences, as he knows Jack will support him in his actions. This incident demonstrates the breakdown of order and the rise of savagery on the island.