Variable geometry pylon

The variable geometry pylon was a feature first introduced to Federation starships around 2370.

The warp nacelles have the ability to be raised into position for warp speeds, then lowered into a more streamlined position when at slower-than-light speeds. The starships were equipped with this feature, examples of which include the USS Voyager and the USS Bellerophon (NCC-74705). (, et al)

The pylons were raised anytime the ship's warp field was at power levels above idle, even if not used for propulsive effect. 

Background information
The Intrepid-class was the only known Federation starship class to incorporate this design feature. The first time the concept of variable-geometry warp nacelle pylons was mentioned anywhere is in the Star Trek: The Next Generation Technical Manual (p. 178) in the section dealing with preliminary concepts for future starships. The goal of these pylons was to improve engine efficiency by optimizing field stress when the ship travels extended journeys at warp 8+ velocity, resulting in significantly improved engine efficiencies.

In the unpublished VOY Season 1 edition of the Star Trek: Voyager Technical Manual, (p. 12) by Rick Sternbach and Michael Okuda, it was suggested that because of this new folding wing-and-nacelle configuration, warp fields might no longer have a negative impact on habitable worlds as established in.

VOY Season 7 episode however suggested that the warp drive still caused the cumulative damage. In the episode, members of the Hierarchy capture Captain Janeway and demand the warp core of the Voyager in exchange for her. The Doctor comes up with a story for the crew of the Voyager, that they had entered R'Kaal space, a species of ecological extremists he had invented, who had supposedly outlawed warp travel in their space, because conventional warp engines damage subspace. When he claims this is the reason why Voyager has to surrender its warp core, the crew makes no effort to convince the R'Kaal that their warp drive is totally environmentally safe.

可変静翼パイロン