Has anyone you know ever used the -G --incremental option successfully with the tar command?

I'm not talking about the little -g --listed-incremental option. I don't want incremental backups. Just one differential backup which is what the -G is for. The -G command is supposed to make a single incremental archive based on the dump archive, when compared to the source directory that the dump archive was originally created from. The incremental archive should contain all files that have been modified since the original dump archive was taken.

Using tar with this option seems like a very prevalent and useful tool for a simple differential backup plan. I have devoured the man file for tar and the GNU tar manual GNU tar 1.19
They don't really say how to use it.

My biggest question is, where in the command do I say which dump.tar file it's supposed to compare to the source directory, in order to make the incremental or difference tar file?

Help is much appreciated.