Answer: To show that it is a metaphor and not literal
Step-by-step explanation:
''Talk the hind legs of a donkey'' is having a meaning ''making a donkey sit down on its rear end'' which is not something that donkeys do naturally. It is used when we are describing a person who is talking to much. This phrase has its origins in Ireland.
In this case, the author is considering that person can talk much about her tales.