it is because of a next generation episode when they found out that warp travel was causing distortion (read pollution) in space and was killing a planet in a high traffic area. They put a speed limit on that area. From then on they redesigned the ships to not pollute. hence the reason why the nacelles move up. the newer ships such as Enterprise E have the design built in or something like that.

geeky enough for ya!

my guess is that the thing between the nacells is engineering
it's positioned perfectly to feed plasma to the nacells

C'mon ya gotta love Phlox kicking ass!

"I've used all my muscles already!"

I still like this show. I guess I'm easily entertained.