and search... 9/11 WTC South Tower Collapses, West Street ...
the uploader is... coolgamer1677
(don't ask.... )
I'm afraid that I'm not seeing any flash. Here's the video link.
Perhaps if you paused the video at the point where you are seeing a flash and took a screen shot? Perhaps compare it with another screen shot immediately before and/or after?
Also, if it is that far into collapse, you are contending that the intention was to disintegrate the tilting upper block so that it causes less damage, right? Why is the upper block still quite visibly fully intact at that point in other footage from different perspectives?