I really like the way Hunter Robertson does this song.
https://www.youtube.com/watch?v=aQikpvdrs00
He seems to play boom-a-chuka-boom-a-chuka boom-chukka-boom-a-chuka
and it looks like he gets the "a" after the first boom by upstroking with his index finger. The first "boom" seems to be just the usual clawhammer downstroke with the index. It gives the tune a VERY pleasant rhythm. At first, I thought it was double thumbing, but I don't think so now.
I hear that rhythm in some of Clifton Hicks' playing also, and I'd dearly love to know how it's done. Can someone give me some help, please? Thanks.