I discovered this thread completely by chance on an unrelated duckduckgo search, but it taught me how to get that thwack on the opening note. Much appreciated TDPRIers. https://www.tdpri.com/threads/guitarist-on-six-days-on-the-road.106427/ Thread spoiler. During a break, I spoke to him [Jimmy Colvard] for about 15 minutes. He said he got the popping sound by having his bridge pickup on and snapping the top of the three bass strings (one at a time) with his middle finger fingernail. The last part of the guitar ride in Six Days also has a downward flat pick string sweep. It's in the key of G. He was very friendly and easy to talk to.