It reuses world space radiance cache to spawn a new grid of probes. Each probe stores depth in order to do visibility testing DDGI style and after tracing is converted into irradiance. This thingy replaces final gather, so instead of using ScreenProbeGather it does final gather from world space probes. Basically a DDGI prototype.
Though it was done like 2 years ago or something, so likely nowadays is completely broken :).