Parkie, You answered your own question - if a surface is hidden from a light source, that means it's in the shadow of something else. In this case, the inner floor of the pipe was supposed to be in the shadow of inner ceiling of the pipe, and therefore dark. But if you turn off shadows, these natural laws are ignored, and the interior of the pipe is "seen" by the light. It's confusing because it contradicts reality, where there's no such thing as not casting shadows.