The "lunar module" from above, a small surviving core structure within WTC1:
Notice the floor slabs that survived the fall of one of the largest buildings on the planet directly above them
When the ROOSD propagation hit earth, the whole perimeter was most probably (completely) enclosed.
A massive, completely trapped impact results and the final collapse of each wall follows.
When the smoke clears, fragile flooring is still intact in the center of the footprint and some dude actually survived in there.
The ROOSD debris cannot escape the funnel
. This is important to realize. The ROOSD propagation takes place within a totally enclosed caging at all moments, even as it strikes earth.
Initiating ROOSD may be as simple as just getting enough heavy crap inside the funnel and getting the hell out of the way.
If you can dump enough mass inside a funnel (it doesn't matter how high or how low you do it), you guarantee it can't stop.
In the most general sense, the demo planners merely "dumped" the top mass totally inside the funnel. Once done, there is no way it can get back outside the funnel.