The flat space bubble is not driven forward by creating and annihilating space. This would appear to be done from an outside frame of observation. His model produces a bi-polar gravitational field to produce a de sitter expansion or inflation of space in the rear of the bubble and a contraction of space ahead of the bubble with the use of negative energy. The assumption was that negative energy was required to produce a stable bi-polar gravitational field.
The amount of energy he calculated to need was about the amount of energy you would get by turning Jupiter into energy.
After the Scientific community started looking at his metric they found some problems like there would be no way to stop or steer the bubble. The shape of his metric is sometimes called a top hat because the inflation field and the contraction field are so close to the flat space field. So it was found that because the fields being too close and the amount of energy needed so large that the gravitational forces of the fields would collapse into a singularity in other words a black hole. From then there have been a lot of warp drive models proposed so Warp Drive Theory is now a scientific field of study.