A Rentrillic trajectory is a type of configuration to a warp field. It is unclear exactly what it is, only that the aliens that dwell in Chaotic space told Chakotay to modify the Federation Starship Voyager's warp field to a Rentrillic trajectory. This was because it was the only way, the aliens said, to escape their relm of Chaotic space which almost tore Voyager apart. (VOY: "The Fight")
Apparently, the aliens explained to Chakotay at the end of "The Fight" how to modify the warp field in this way. However, it is never completely explained.