Question about the end of the Xiao Yu segment ***spoilers***

I didn't think too much about that, felt like a build-up to her retaliation. Before she did what she did she seemed like a pretty calm person we kept enduring the events she experienced, this was the boiling point for her. I think while she was repeatedly being hit she was deciding if she wanted to finally strike back the way she did. This event was loosely based on an actual incident (according to the Wikipedia page, all four events were loosely based on actual incidents http://en.wikipedia.org/wiki/A_Touch_of_Sin. The woman was not mentally well.

Agreed. It was at that point in the film, I was thinking "Ok, we get it. We get the message you're trying to send." It was a little ridiculous how long it went on for. But maybe the entire movie is like a weird meta of that scene. The director repeatedly hits us with these stories that very obviously illustrate the corruptive influence of money on society.

People in power expect others to bow to their will. Bow, bow-down, turn-away, submit, etc. By returning to her feet, at first, and then turning back to look her attacker in the face, she refused to submit to his will. She is saying, "You are not better than me!", "I will not submit to you!", or any other words of defiance.