None really. They fold space. That's all that is really said about it from Frank Herbert's works.
Dune is a book about politics within a layer of science fiction. It isn't meant to be a "how did they make a ghola?" (though that is explained in some of the prequels). Or the actual biology of wtf did he do with that sand trout?!
As described in wikipedia on soft science fiction:
Frank Herbert's Dune series is a landmark of soft science fiction. In it, he deliberately spent little time on the details of its futuristic technology so he could devote it chiefly to addressing the politics of humanity, rather than the future of humanity's technology.
Don't worry about it. FTL happens. It is a tool of political control - not a piece of technology to be worried about.
Trying to understand fold space in the context of the Frank Herbert Dune novels is counter to the actual ideas and goals of the author and a distraction to the reader.