Notice the two puffs of white smoke, or "squibs," coming out of the South Tower as is starts to collapse.
The bottom squib is the one in question. Notice how far down it is from the demolition wave. Notice it casts a shadow. Also, notice in the video below that this bottom squib starts before the top squib.
So what is causing that puff of smoke far below the demolition wave? "Compressed air"? Or is it proof that explosives were used to bring down the Twin Towers?