  We show that there is a correspondence between totally umbilic null
hypersurfaces in generalized Robertson-Walker spaces and twisted decompositions
of the fibre. This allows us to prove that nullcones are the unique totally
umbilic null hypersurfaces in the closed Friedmann Cosmological model. We also
apply this kind of ideas to static spaces, in particular to
Reissner-Nordstr\"{o}m and Schwarzschild exterior spacetimes.
