Here's the NIST theory.
NIST's final Report on the Twin Towers mentions the
piston theory to attempt to explain away the ejections:
The falling mass of the building compressed the air ahead of it, much like the action of a piston, forcing material, such as smoke and debris, out the windows as seen in several videos.
There are several problems with this explanation, which we designate the piston theory.
9-11 Research: Squibs
Not that I subscribe to it, but carry on, as the term "piston collapse" is a mechanic's term which has been erroneously and conveniently taken up by NIST to explain a compression effect that they theorise occurred as each floor fell, so forcing puffs of dust out of the sides, leaving a much degraded outer shell and still standing, as in a volcanic caldera, a group of central core columns.
The trouble with the "collapsed piston effect" theory is that smoke from the explosive charges, "puffs of dust" that NIST so quaintly puts it, were heard and observed blowing out
several floors below, and in advance of, the collapsing floors above them, so a no runner NIST theory there.
Neither the pancake theory nor the so called "collapsed piston" effect fit, as each tower would have taken around 64 seconds to collapse in a both theories, due to resistance inherent on each floor impacted from above and the distance between each floor, so the acceleration of the mass as it fell would have been much impeded, taking the towers far more time to collapse than the approximately 10-11 seconds recorded that day.