Answer:
Tan winces when she describes her mother's English as "broken".
Step-by-step explanation:
Tan states that she has trouble describing the English her mother uses, the only way she can describe it is as "broken". But when she does this, she "winces", this means this hurts her, and that pain is visible in her face. We can say Tan doesn't have the sufficient linguistic resources to explain in a correct way what she thinks.
As she tells us, she doesn't believe her mother's English is broken, or damaged. She wants to state that although it's different, it is as valid as regular English.