Essentially a furled leader is a different type of knotless tapered leader which is made in a way similar to some cord and rope.
All sorts of material have been used, most commonly thread and monofilament.
The art of furling the material together looks quite involved, but I'm told it's simple once you've got the basics nailed. Have a look here:
Characteristics
Good and bad, but the good is turnover tends to be gentle encouraging good presentation. The bad
They're a bloody pain if they so much as get a whiff ob being tangled.
I stopped using them some years ago, but I would probably choose to use one again if I were fishing rivers for trout more often than I do.