Subject: How does the body of a whirlygig affect the time it takes to fall?

Paper helicopter/ whirlygig/ whirlybirds: How does autorotation work and how 
does the body affect this? Also, how does the body affect the rate the whirlygig 
falls?

I've recently become very curious with toys called whirlygigs/whirlybirds/paper 
helicopters. These toys are not often used and so I cannot find much information 
about how they work. Here's how a paper helicopter looks 
like:http://www2.mae.ufl.edu/haftka/eoed/Project_files/image002.gif or 
http://www.stephanieblakey.me.uk/models/Simple-Helicopter/Helicopter-thumb.jpg

So far, I've managed to learn that paper helicopters are more similar to 
gyroplanes/autogyros/gyrocopters rather than helicopters and that the paper 
helicopter spins to the ground using a process called auto-rotation.

What I am confused about is what auto-rotation is and how it works. It doesn't 
make sense that because of air resistance the blades spin. After all, shouldn't 
air resistance simply lift the blades up? Why does air resistance turn the 
blades? Also, I've found that paper helicopters generally have a square body. 
However, in some models, the body is angled (the first picture of a paper 
helicopter has a square body whereas the second picture has a more trapezoid 
body). I've tried making both models and found the more angular model to fall 
more slowly, but I am uncertain as to why this happens. How and why is it that 
the body affected how slowly I saw the paper helicopter fall to the ground?

Re: How does the body of a whirlygig affect the time it takes to fall?