Watching Martin Miller again, and it looks like he is more or less throwing his arm up and down (don’t know how to describe this in other words) from the shoulder to get this particular flow going on for the one note per string stuff. Within this flow he then puts in the fine picking movements.
This movement has to come from the shoulder in my opinion, because the whole arm moves a bit towards the neck side.
If it came from the ellbow the arm would move to the bridge side.
It is stringtracking of course, but in a way i have not see any other player do.
Even when playing just on one string this movement is still a bit present.
Look from about 6:32 in this last vid Troy did with Martin, it is very obvious here: