The Dmitrius dude has no idea what he's talking about. He likens the pancake effect to 1 car hitting several cars or a couple cars being dropped onto a stack of cars, neither of which is in any way related to the pancake effect.
Each floor of the building would have a force due to gravity and an equal magnitude normal force exerted by whatever is keeping it stationary. Let's assume for argument sake that all of the floors weigh roughly the same which means those forces are the same on each floor.
So each floor weighs n kg, which means each has a gravitational force of (-9.8m/s^2)*n kg and a normal force of (9.8m/s^2)*n kg.
The first floor drops. So from kinematics we know its velocity at any given moment in free fall will be v = (-9.8m/s^2)*t so at 1 second v = -9.8m/s giving us a momentum of -9.8n kg*m/s.
Let's say the top floor hits the 2nd from the top at 1 second and barely comes to a complete stop over the next second (because easy math). So using our equation F=mdv/t we get roughly:
F = (-9.8m/s)*n kg/1s or F = -9.8n kg*m/s^2.
Add that to the force of gravity already acting on the floor, and we're at F = -19.6n kg*m/s^2. If that is more than the supports can supply in normal force, then the second floor collapses.
Now the falling structure weighs 2n kg. So after an additional second it will have a momentum of -19.6n kg*m/s. If it applies that force over 1 second again, we now will have a total force of
F = -19.6n kg*m/s^2 which added to the existing force of gravity gives us F = -29.4n kg*m/2^2. Since we said that support structures at 19.6n kg*m/s^2 would be what the normal force could slow to 0 before breaking, with this amount of force the falling debris will not go to 0.
So what we get is a falling structure that weighs and extra n kg with every floor and has a linearly increasing momentum with a proportionately increasing force, resulting in less deceleration due to the normal force of the floor below it. So unlike the car example where the momentum would be distributed among the stacked cars, here due to the constant acceleration due to gravity and the constantly increasing mass we get more of a domino effect with the overall force on each floor increasing as the fall progresses.
Of course, this is a vast oversimplification. The floors would all weigh different amounts, and the normal force the support structures could supply would be affected by partial collapses and fires and any other factors that would have damaged structural integrity of an individual floor. Plus, the amount of time spent accelerating from one floor to the next and the time over which these forces were applied would have changed throughout the collapse.
But in general, the idea that the floors on the bottom would have somehow been cushioned by those up top slowing the collapse is just plain wrong. That would only happen if they were stacked without any time for the floors to decelerate in between, so that the normal force would be equivalent to the combined force of gravity on all of those floors on the ground. As these floors were suspended, they had to each individually absorb the force on their own rather than taking it as one unit.
