OK, here's my best description of why the rotors look bendy.
Shutters on cameras actually consist of two moving curtains (at least all the ones I've seen).
When you press the button the first curtain moves down, and is then followed by the second curtain. The time before the second curtain follows the first is set by shutter speed (fast shutter speed = smaller time = smaller gap between curtains = less light to film). Thus to get same amount of light to the film you can have a small lens aperture (not much light gets through the hole) and a long shutter speed (to get more light onto film), or a large aperture (lots of light gets through) and fast shutter speed (limits light onto film). How this is set up depends on what you taking photos off, and how you want them to look.
As the curtains move with a gap between them, often the whole frame is not be exposed at once, only a 'band' across the frame, that moves down as the curtains fall. Thus when the exposed area is at the top of the frame, the blades are in one place, when the exposed area moves down, the blades have also moved. By the time the exposed area reaches the bottom of the frame, the blades are in a completely different place to when the exposed area was at the top. Thus the moving of the exposed area and the blades make them look bent. This also happens with the main rotors on some shots.
With my SLR if I set a slow shutter speed the rotor discs are just a blur, but if I set a high shutter speed I can get them perfectly straight and static. If I mucked around with a few different shutter speeds in between, I'm sure I could achieve the 'bendy blade' effect.
The above is all based on what I've picked up along the way using my SLR camera, if I'm wrong on any points please correct me.
I hope the above is fairly clear
, you can probably see why I'm not a teacher, 'cos I'm crap at explaining stuff (oh yeah, and I hate kids)!
(edited for spolling miskates)