This has come up several times on the user's list.
On Windows, which prevents deletion of still-open files, IndexWriter cannot remove files that are in-use by open IndexReaders. This is fine, and IndexWriter periodically retries the delete, but it doesn't retry very often (only on open, on flushing a new segment, and on committing a merge). So it lacks immediacy.
With this expert method, apps that want faster deletion can call this method.