|Watch this video.
Near the top floor, you see a guy waving a white shirt (at least it looks like this). He's in the upper right corner of the building that's facing the camera, just below the dark area, and just at the top of the uniformly gray part of the building. Just when WTC2 is hit, the pigeon in the foreground flies away. Right at that moment, a bunch of things happen.
You are looking at the North face of WTC1.
a) That little red flame on the right side of the West Face of WTC1 explodes out, then returns to "normal."
b) There is an explosion, a puff of smoke, that bursts out the window of the guy waving his shirt. After that, he is no longer waving his shirt.
c) There are multiple puffs of smoke that shoot out of the North face of WTC1, on many floors between the plane's hole and the top of the building.
d) Right after the fireball (and camera shake) it looks like 2 people jump out of the lower floors of the smoking region of the West face of WTC1. This is just above and to the right of that little red flame.
e) Then, there is a large puff of smoke that explodes out of a window, about half way up the uniformly gray region of the building, on the very right side, between the upper right plane hole and the dark top of the building.
f) Then, a few seconds later, here comes the chopper, flying around the back side of the towers. I think this chopper may have been there to set off these detonations, just as WTC2 was hit. Perhaps they assumed it would be less noticeable. After all, the explosions you heard were of the fuel exploding out of WTC2, right? ...sure...
g) Just when the chopper is midway across that blue patch of sky, notice all that huge amount of smoke shooting out of the left side of WTC1, from the roof and from the East face. The smoke keeps shooting out for the remainder of the video.
h) Just after the chopper is out of the picture, the guy's shirt falls out of that window.
There are plenty of other things, but these are enough for now. :-]
Here's what I think:
I believe there were huge explosions, during the time of this video, causing a lot of air pressure to push through the building. Perhaps it is the open windows that allow the venting. But, it's interesting that it doesn't shoot out of the hole where the "plane" went in. So, perhaps the explosions were in many of the floors above the hole, which were to prepare that top chunk to disintegrate, later. My guess is that those people jumped, thinking the building had just blown up. Their only chance for survival would be to jump clear of the exploding building.
Hey, notice the pigeon flies away *before* the big WTC2 explosion. i.e. The plane hit didn't jar the building much. The explosions shook the building. The plane didn't! I'm assuming the plane hit about the time the bird flew away. The bird would fly, sensing danger. He's going to fly away as the building is hit. Notice he glances over at WTC2, several times, waiting to see if he needs to leave. He sees the plane coming.
Note, "explosions" are not necessarily from "explosives," but could be the result of something else, such as the dissintegration of pressurized tanks.