Did find this, which shows the construction of WTC7. It appears that the upper portion of the building was essentially supported on a framework that left alot of clear space at the bottom of the building. I think the end of the clip is trying to suggest that if you blew the internal columns on each of the floors (with the invisible explosives, placed by invisible workers, that could survive in a major fire presumably) that the building would collapse in the way that it did, though if the lower structure fails you would get the same effect and it doesn't need many columns to fail in the lower area.