Thanks, Will, that's a start. I think the entry is a bit weak on information (as you suggest), not written by someone who really understands, but it gave me the tip that part of the the purpose of re-entrant tuning on banjo is to play clawhammer, a term I know without knowing the meaning of it.

For anyone interested, here is a video explaining clawhammer banjo. And here it is put into pratice on the tune Kitchen Girl.

You could do the same on ukulele, I think?

Rather different on therobo, as the 'out of synch' string isn't at the bottom. Instead, the top two strings are down an octave. How does that work? And why?