The warp nacelles have the ability to be raised into position for warp speeds, then lowered into a less streamlined position when at slower-than-light speeds. The Intrepid-class starships were equipped with this feature, examples of which include the USS Voyager and the USS Bellerophon. (VOY: "Caretaker", DS9: "Inter Arma Enim Silent Leges" et al)
The first time the concept of variable-geometry warp nacelle pylons is mentioned anywhere is in the Star Trek: The Next Generation Technical Manual in the section dealing with preliminary concepts for future starships. The goal of these pylons is to improve engine efficiency by optimizing field stress when the ship travels extended journeys at warp 8+ velocity.
In the unpublished VOY Season 1 edition of the Star Trek: Voyager Technical Manual, by Rick Sternbach and Michael Okuda, it is suggested that because of the variable geometry pylons, warp fields might no longer have a negative impact on habitable worlds as established in TNG: "Force of Nature". According to the VOY Season 7 episode VOY: "Renaissance Man", Voyager's warp drive is still a space-time fabric harming conventional warp drive.