Forward explains that he found the Tunguska meteorite, which was actually a small black hole. Returning it to the station he fed it the sphere of neutronium he was previously using for his communications attempt, thereby increasing its mass, and then fed in the exhaust of an ion engine to give it a permanent electric charge. The hole could now be manipulated with magnets, and towed around by the tugs. The tugs move it into the path of incoming starships to disable them, and then pirate the now-defenseless ships.