After reading the paper, it would seem that for null mass particles (which I assume refers to light particles), when the warp bubble is traveling above the spee

After reading the paper, it would seem that for null mass particles (which I assume refers to light particles), when the warp bubble is traveling above the speed of light, there is a point towards the front of the bubble where light particles travelling in the same direction as the bubble are swept up and held (a front horizon). As the bubble decelerates below the speed of light, the front horizon moves forward outside of the warp bubble, at which point these particles are released ahead of the ship at their regular speed of light, but with an observed energy exponentially increased in relation to the time they spent in the bubble. A similar effect is expected with massive particles moving at near the speed of light in the direction of the bubble's travel, which can apparently be accelerated to a speed nearer to that of light upon exiting the front horizon.