And he's frailing a banjo! Typical style for Old Tyme Music, but Bill would fire any banjo player that did that.
I noticed that too. And if you, dear reader, are wondering what Ol' Nickie & I are talking about, take a look at this puff for Wayne Erben's instruction books for banjo:
Bluegrass or Clawhammer Banjo - Which is easier to learn?
"Oh, but that's banjo - how do I apply these insights to ukulele?", I hear you cry. Well, like a ukulele, a 5-string banjo has re-entrant tuning. The top, 5th, string on a banjo, is a 'drone' string, never fretted, but just plucked to provide a driving rhythm.
To apply either the frailing (aka clawhammer) or bluegrass technique to ukulele, you need to use the top, 4th, string (g on a gCEA-tuned uke), as the drone string.
Clawhammer rhythm is "bum dit-ty", (
bum? pluck any string but 4;
dit? strum strings 1,2,3;
ty? drone - thumb-pluck 4).
If you want to learn more on clawhammer for uke, there are
lots of videos out there explaining how to apply clawhammer banjo technique to ukulele using the 4th string as a drone (though one thing
none of them mention, however, is that to play clawhammer uke, your strumming hand has to hit the strings on or about the sound-hole, not over the fretboard).
I don't think there's that many on bluegrass style for uke, so you'll pretty much have to try to modify the banjo tutorials for uke.
Bluegrass rhythm, according to Erbsen, is "thumb-pinch" (
thumb? pick a (melody) note;
pinch? play strings 1 & 4 together).
If you want to read further, you could do worse than start with
What Are Bluegrass & Clawhammer Banjo Styles?.