How does Crooks avoid Curley's wife's attack?

"Crooks had reduced himself to nothing. There was no personality, no ego - nothing to arouse either like or dislike. He said 'Yes, ma'am,' and his voice was toneless.

"Crooks sat perfectly still, his eyes averted, everything that might be hurt drawn in."

He becomes withdrawn and characterless, always answering her in the affirmative, so that she can't find anything to attack him for.

"...retired into the terrible protective dignity of theNegro."

This seems to be a well-practised move as, as a black man in Southern America, he has had to find ways to protect himself all his life and so has learnt how to avoid attack by not answering back when he can sense danger.