I Cannot Sit On My Heels

This morning on the bus, I was sitting behind a woman of Asian descent who was apparently studying English. The piece of paper she was reading had two columns, the first with a sentence in English, the second with a sentence in what appeared to be Chinese or Japanese. This is not all that unusual. What was unusual, however, were the sentences she was learning how to say. The first four on the list were:

That is so humiliating!
This law violates the First Amendment.
Her back was turned when she was shot.
I cannot sit on my heels.

Say what? I cannot sit on my heels? What does that even mean? And why would this woman need to learn how to say it? She might as just run around saying "I can eat glass."