In part 5 at 1:30, Alan is talking about playing in the key of G and then going to an A major chord, which happens in some bluegrass songs. Is he saying that you are temporarily crossing over into a different key, the key of D?
I know that when I’m playing the Old Home Place in G, the chorus goes D->G->A->D, then to verse --> G -> B7 -> C -> G
The A major chord is sort of unusual and unexpected, but sounds really cool in the chord progression. I can see how if you are crossing over into the key of D there, the A is the V chord in the key of D, which then wants to resolve to the I chord, which would be D. And then the D chord is common to both the key of G and the key of D, so it gives you a way to cross back into the key of G?