This style of 'a capella' singing is new to me. It is called Sacred Harp singing or Shape Singing. https://en.wikipedia.org/wiki/Sacred_Harp
I had not heard of it until I came across a link in a comment on a US blog a couple of weeks ago. The link was the first video shown below. I found this brief description beneath one of the other videos in the series but don't remember which:
"Welcome to the incredible talent of the Sacred Heart Singers of Cork, an Irish music group who sing a cappella in the style of Sacred Harp, a sacred choral music that originated in the American South of the United States.
"Having originated in the South in the late 1700s and early 1800s, the singers are led not by traditional musical notes but by shapes which represent the different pitches while time is not kept with a formal conductor but through the communal keeping of time as everybody beats their hands."
The tune and lyrics of Babylon Is Fallen are thought to be from the 17th century and it then travelled to the USA with the Shakers. In the USA it is known as Shape Singing and, by coincidence, that phrase appeared in the new BBC series on the origins of country music.
The first of Ken Burns' films told the story of the first recordings made in Bristol, Tennessee in 1927. One of the singers had borrowed the Shape Singing hymn book from a local Methodist church and had adapted some of the tunes/lyrics for his recording sessions.
Apart from the brief notes above it is difficult to find much information on this Sacred Harp or Shape singing but there is a lot of it on YouTube.