The idea is that a nuclear pulse propelled spacecraft can be made far less massive by using cables in tension and a magsail rather than a huge pusher plate. The forces are much less severe, being in tension. The cables are stronger in tension, and the elastic elongation depends on length. The magnetic field (if large enough) can recover far more energy from each nuclear detonation than a relatively tiny pusher plate. Basically you get a smaller, lighter, faster starship design for deploying your deep space probes (synthetic aperture telescope interferometers, and the like).

Apparently the Brits came up with the idea a while back. Excuse the image quality (didn't put much thought into it).