Apparently, Sacred Harp (also known as Shape Note music), originated in the colonies and then migrated south, where it took root, especially in the Appalachian areas. The "hand chopping" action is used to keep time. Because there are no instruments accompanying the singers, the names of the notes are sung, i.e. "Fa" "La" "So", etc. This was so untrained musicians could still sing, reading the "shapes" of the notes rather than musical notation.
When there are lyrics, the words are sung, and what words they are! They are very old-school primitive hymns, which I normally don't find very meaningful, considering I've grown up in a pretty contemporary praise and worship-style church my whole life. But there is something raw and real in this music. The emotion echoes around in the old wooden churches. No organ, no microphones, just voices. I can't listen to it without tearing up. You can learn more about Sacred Harp here.