The “secret sauce” behind that is the fact subsequent results are generated primarily through multiplication and not raytracing.
For anyone interested in knowing more: SAXENA, Mudit et al. DYNAMIC RADIANCE – PREDICTING ANNUAL DAYLIGHTING WITH VARIABLE FENESTRATION OPTICS USING BSDFS. Proceedings of SimBuild, [S.l.], p. 402-409, aug. 2010 Link