Jefferies appeared to consider the rings to be the warp drive. There is no other apparent drive on his various ringship designs. Also, it fits with modern conceptions of a hypothetical "Alcubiere" warp drive requiring a negative energy induction ring. Finally, we are told a nacelle is filled with multiple ring-like "warp coils". It would make sense that an early warp drive might consist of only one or two of those coils.