After watching the vid multiple times, and a bit of experimentation, here's what I think is going on, for what it's worth.
First, the guy uses a sort of piper's grip / fluter's grip on the right hand, playing with the 2nd joints, which disguises the fact that ...
... what we are seeing, I think, is just a playing affectation, and not an actual ornament. When he executes an upward slide from, say, an F# or E to a note with the right hand completely off, he just tends to come off the note by sliding his hand down as a kind of physical flourish.
You can't hear any kind of stuttering ornament, because there's nothing to hear. He even does it when tapping the F# in G rolls, which makes it seem like there's something more there. But there isn't. It's just a G roll.
Think about it. On most whistles (including a Gen), the only bottom hole that affects any of the top hand fingerings strongly enough to produce an audible stutter is the B1 hole when ornamenting a G xxxooo. Otherwise, rubbing 1, 2 or 3 fingers across any of the bottom holes does nothing for any top hand fingerings. Nada. It's just useless movement.
Conversely, bouncing your fingers 3-2-1 on the open hole below any left hand note (like the B1 hole while playing G) definitely produces ... something. Rather like a triplet on a piano accordion. A triple tap. The same goes for bouncing on T1 while playing c#. I guess that describes Steafan Hannigan's technique. The problem is then getting your hand back into position quickly enough at speed to continue playing. Seems more like a parlor trick than a useful technique.
But as far as I can tell, that's not what the YouTuber is doing anyway.
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