“I’m saying that the Xeelee can create, and control, space-time defects. We think that the ‘wings’ of this nightfighter are defects—domain walls, bounded about by loops of cosmic string.
“Spinner-of-Rope, the Xeelee use sheets of antigravity to drive their spacecraft...”
The domain walls were inherently unstable; left to themselves they would decay away in bursts of gravitational radiation, and would attempt to propagate away at speeds close to that of light. The Xeelee nightfighter must actually be stabilizing the flaws, actively, to prevent this happening, and then destabilizing the flaws to gain propulsion.
-Ring, Ch. 17