The net was an instrument of particle manipulation and envelopment developed by the Outside Enemy, Daniel and Marty.
When the captured gholas of Duncan Idaho, Miles Teg, Scytale; Sheeana and her ultra-orthodox Bene Gesserit sisters left Chapterhouse, the elderly couple Daniel and Marty tried to ensnare the no-ship with their net. The net appeared as a series of non-organic gossamer strands that slowly wrapped itself around the ship, getting stronger as time passed. Eventually it would have encompassed itself completely around the giant structure and trapped those inside, had not Duncan Idaho folded space to escape.
Behind the Scenes Edit
In Chapterhouse Dune "Tachyon theory" is mentioned in connection with Holtzman's "original design", however he calls his power source techys, not tachyons. Duncan concludes that "someone 'out there'" has discovered a new application for Holtzman's theories - this could imply the existence of a Tachyon Net, however it is never mentioned by that name by Frank Herbert.