Take a look at the source for a hint about what it does.
Another hint, ... the API says "This method differentiates RollingFileAppender from its super class." The parent of RollingFileAppender is FileAppender. So, what would be different in RollingFileAppender from FileAppender ?
When is it called? You can find that in the source.
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Building a Better World in your Backyard by Paul Wheaton and Shawn Klassen-Koop