Dereferencing hard links

In the manual page of tar command, an option for following hard links is listed. -h, --dereference follow symlinks; archive and dump the files they point to --hard-dereference follow hard links; archive and dump the files they refer to How does tar know that a file is a hard link? How does it *follow* it? What if I don't choose this option? How does it **not** hard-dereference?