I remember reading in the art book or an interview somewhere that the Jellyfish's warp drive was based on "new principles". So not an Vulcan warp ring, not a Cochrane nacelle pairing, but something else. Something faster.
As for it spinning at sublight speeds... don't the TNG-era warp engines glow the same sublight as they do at warp? It's also possible that this drive works as well at sublight velocities as it does at FTL, eliminating the need for separate impulse engines.
The art book also talks about the skin of the ship being organic and grown over the frame, which is pretty cool.