This is how I have understood it:

  When doing radiosity calculations povray stores color values in a spatial
tree. It also reads the spatial tree to see if there was already a value
calculated for a point in space near to the current one (if it's close
enough, then it just takes it and doesn't have to calculate it).
  If there are many pre-calculated values in the tree, then the final pass
doesn't have to calculate so many radiosity values and it's faster and
more accurate. For the same reason the second pre-trace pass will be faster
and better than the first one, and so on.
  The more the pre-trace passes, the faster and more accurate the final
render (but also the slower the pre-trace stage).

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):5;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

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