[mattdw]

It's essentially the same process to begin with (I think), but every time the ray hits something you fire off secondary rays probabilistically from the hit point to see what that point is receiving light from, and so on ad infinitum. Usually you're calculating a BSDF[1] for each surface you hit, which tells you where to cast your next set of rays.

[1] https://en.wikipedia.org/wiki/Bidirectional_scattering_distr...