Have you ever heard of using keys for self-defense?

“Walking home to Euclid Avenue in the evenings, I carried my house key wedged between two knuckles and pointed outward, in case I needed it to defend myself.”
—Michelle Obama, on growing up on Chicago’s South Side; in Becoming (2018), p. 72

I grew up in the suburbs of a major U.S. city and later lived in the city. I never heard of that trick until I read of it for the first time in this book.

Have you ever done this, carried your keys this way? Have you ever had to use it in self-defense? If so, did it work?