I haven't read the writing or speech, "ain't I a woman" but from background knowledge, I know that rhetoric and rhetorical devices are used to persuade the audience of something.
As I don't know what the speech is about, I couldn't tell you what makes it effective but your answer is going to most likely be a type of rhetorical device.
Ask yourself...
What does the speech use more of?
Alliteration
Hyperbole
Metaphors
Similes
Those are some examples of rhetorical devices. I know I haven't read the speech but I hope I was at least some help :) Let me know if you have anymore questions.
