Extract one folder from a larger tar archive
What just happened to me is that I managed to delete the emails from my Maildir folder. No biggie, I have everything backed up. But the backup is 14GB. I don't want to extract all that so here is how I pulled out a single folder from the huge tar archive.
tar xfz cur.tar.gz 'relative/path/to/the/folder/without/leading/slash'
This worked because my archive was created with relative paths in the first place.