According to some things I've read about field mechanics is that two fields that move against each other will generate energy. Bring the nacelles briefly toward each other may be part of the reason why Intrepid is considered smart and effiecient. It may not need as much power to jump to warp in the first place as say the Galaxy or Excelsior Classes. That may also be why Intrepid has the smallest engines to hull size ratio than any other ship.
Speculation of course.
But the nacelles don't power up until they are actually in the full up position. The warp charging does not occur during the motorisation period.