Beneteau in mast furling
One of the big appeals to in-mast furling is that at the end of a sail you do not have a sail to flake (fold on the boom). With slugs (slides in the mast) folding the sails on a small boat, say under about 33-35 feet is not too hard, and if you are in good shape its no big deal to flake a sail by yourself but as boats get larger, or people are less physically able, it gets harder to flake a sail on the boom.
The Dutchman is a patented system that flakes the sail on the boom as the sail comes down. It consists of a series of vertical wires or monofiliment that pass alternately through grommets in the sails. Since the wires are tight and pass back and forth from one side of the sail to the other, as the sail lowers it folds in neat flakes of near equal size. In many conditions it is simply then a matter of tying on a few sail ties, and putting the sail cover on. Like many other options it complicates things a bit and the system is not terribly cheap, but it allows the roach (the aft curvature of the sail) to be cut normally and to use battens, both of which increases performance and the battens help extend the life of the sail. One negative to the Dutchman is that the folds are always on the same line fold after fold and this can take a toll on the fabric on those lines.
An older system is called lazy Jacks. Lazy Jacks are a series of small ropes that forma a simple spider web of sorts on either side of the sail and gather up the sail as it is dropped. Lazy jacks do not pass through the sails so the sail is not folded as it is lowered. The nice thing about lazy jacks is they can be taken out of the way so they don''t chafe the sail on a long passage. The bad news about lazy jacks is that they make it harder to flake the sails neatly and so sails are often tied off all bunched up with also greatly shortens the life of the sail.
I hope that helps explain the Dutchman.