It's when you have an open hole followed by one or more closed holes like on simpler instruments like recorders and early clarinets.  Forked fingerings usually sound weak and often have tuning issues.  Modern instruments' mechanisms are designed to eliminate the need for many of them (e.g Eb/Bb side key) and improve most of those that remain (e.g. fork B/F#).

And before you ask, you could very well argue that the the long Eb/Bb is a forked fingering too.

Very nice!

I used to do similar tunes.

This is me on clarinet, ukulele, digital piano and double bass samples about 10 years ago.

https://app.box.com/s/ei7542s3umx1ajotjny8

Actually, now that I look again, the upper joint is Boehm system too.  Long Eb/Bb connexion, all four side keys (missing touchpiece on the top one) and the Boehm throat F/F# mechanism.

I had never realised they were like that.  I'd always assumed they had a forked F/C like a sax.

It makes it easier to mic.

A bit over the top that tool.  I used to glue a bit of cassette tape to a matchstick and I used the actual tape not the leader.  I think it was thinner than the leader but I may be wrong.  Maybe I just had cheap tapes.

Now that cassette tapes are relatively rare I just cut a similar width strip of cigarette paper.

I use an ultra-fine cross cut Japanese dozuki saw to cut the end off clarinet reeds for my tarogato.  It does a nice neat job but I only use it because I happen to have one.  It wouldn't bother me in the slightest if it looked a bit messy so long as it worked.  I do get a bit fussy about how some things look but not the arse end of reeds.

But who's playing the music?

If anyone's interested it's Mate Bekavac, a new name to me but wow!

https://www.youtube.com/watch?v=GIl0UyXZcUs

Nice playing.