While they aren't very specific about how their process works, from reading the site http://markforged.com/ it seems that they've produced a special patent-pending extruder head that will pass a continuous thread of carbon fiber through itself. Presumably, this thread is coated with plastic so that it adheres where it's deposited. Apparently it will also work with regular filament, so parts can be made with the composite where strength is required, and filled in with a honeycomb of regular PLA or whatever.

Andrew Werby
www.computersculpture.com